Sorry if any of these questions have been answered already.
 
1. I have a NVIDIA nForce 680i SLI motherboard and was planning on upgrading my videocard (8600gt).  Will PCI express 2.0 X16 cards work on the 680i?
 
2. If they do work would i see better performance running 2 Geforce 8800GT(or GTS, im not sure which is better)in SLI.  Or will running 1 GeForce 9800 GX2 yield better performance?  
 
3. Does the 9800 have any special features id be missing out on if I didnt get it?
 
4. Also  I just wanted to double check and make sure the Intel Core 2 Duo E8X00 Wolfdale CPUs will work with my motherboard with no problems, and do you guys think I would see much of a performance boost going from the Intel 6750 CPU?
 
Thanks for the responses!!

2 8800GT should be right about on PAR with 9800GX2.   The gx2 will probably do a better job of moving hot air out of the case.
 
You'd see some performance gain going to new Wolfdale series, but I think for gaming it would be better to hang on to your 6750 (I use it with my 3870x2 at stock speeds and I'm able to max out every game I have sans crysis at 1680x1050), and wait for Nehalem.

1 - Yes.  PCI-E 2.0 cards work on PCI-E 1.0 and 1.1, and PCI-E 1.0 and 1.1 cards work with PCI-E 2.0 slots.
 
2 - Works like this.  9800 GX2 > 8800GTS > 8800GT.  Generally speaking, the 9800 GX2 is basically two 8800GT's put into a single card, so if you wanted the best performance, you'd SLI a pair of 8800GTS cards, though there are other, and better options considering how far prices have dropped lately.
 
3 - Not sure if Wolfdale CPUs work with 680i.  I think they do if you update your BIOS first.  But seeing how you already have a highly clocked C2D, it's not worth getting a Wolfdale.  You wouldn't get a significant gain.

2 8800gt will provide about equal performance as a 9800gx2. Both of these will shove a fair amount of heat into your case.
 
(2 8800gts) or (2 9800gtx) I believe will give slightly greater performance than a single 9800gx2. They could also be overclocked much higher because each card is dumping hot air outside the case via their dual-slot coolers.

I'd go with the 8800GTS 512's in SLI, and more RAM myself. Just make sure you have good airflow. As for the CPU, you wouldn't notice much of a difference between the one you have, and the one you listed. (or just say screw it, and get a pair of 4850's and a new MB for X-Fire)

EVGA 680i's need the P32 BIOS update, other manufacturer's have their's.  Make sure that you do it before you get the processor.  I have an E8300 on a 680i and it works great.
 
As for the video card, zipzoomfly.com had a MSI GTX 260 for $250 MIR, that's what I'd get and plan on SLI later on.  Also, the 680i boards can do tri-SLI, you just have to buy the bridge.

A pair of 8800GT is about equal to the 9800GX2 and the GTX 280 for less money.  Nice option for SLI chipset owners.
 
As far as the CPU, I wouldn't even bother.  Just OC your e6750 instead of buying and e8400 if you desire more CPU.

Thanks for all the responses!
 
So am i correct in thinking the following?
 
-2 GeForce 9800 GTX in SLI will give me slightly better performance, but ill lose the ability to expand
 
-1 GeForce 9800 GX2 1GB will be a few bucks cheaper, nearly as good performance, and can get another for SLI in the future
 
Im looking between these 2 cards right now:
(2 for SLI - $378) http://www.newegg.com/Product/Prod [...] 6814150292
(1 - $369) http://www.newegg.com/Product/Prod [...] 6814150284
 
Are there better ones out there for the money, or is there a more efficient/cost effective route to getting this level of performance?  My budget is $400.
 
Just to make sure my system will work with this stuff and there wont be any limiting factors my complete current setup is:
 
Antec P182 ATX Mid Tower Computer Case  
Antec True Power Trio 550W Power Supply  
EVGA NVIDIA nForce 680i SLI Motherboard  
Core 2 Duo E6750 2.66GHz  
EVGA GeForce 8600 GT 256MB  
OCZ SLI-Ready Edition 2GB (2 x 1GB) SDRAM DDR2 900/OCZ Platinum Revision 2 1GB SDRAM DDR2 800(Soon to be upgraded to 4GB of DDR2 1066)
 

The 9800GTX SLI looks like your best bet (and by the way you can usually only get the MIR on one of the same item).  Especially since it's XFX and it's overclocked, it should do you pretty well.

Good point, I didnt even think about only being able to get 1 MIR

That power supply is on the border of being able to handle the cards you'd like to run.  
 
You can definitely run 1 98009800gtx, but there's no way you could run two.  They each Require 2x-6-pin connectors.  It would take at least 650W, probibly more, to SLI 2 9800's
 
As for the 9800GX2, it requires a singel 8-pin connector, so make sure it comes with an adapter since your power supply doesn't have one.  You could probibly run it on your 550W since it's a good unit and you're running a dual core processor, but It will strain it, and may lead to a short life.   A 580W unit is recommended.  
 
I would go with a GT260 in your shoes.  It's probibly the best you can do within your budget, and power restraints.  I saw one the other day for 269.99 after rebate.  It's a ton faster than a 9800gtx, and uses about the same power.  In the future you could add another for SLI.  And, if you OC it, you can probibly match the 9800gx2 performance.  I'd go easy on the OC'ing though until you figure out the power strain it's dealing.
 
The GT280's are getting down around $400 with rebates, but they draw more than your PSU can probibly handle.

If I decided to instead of getting new RAM, get a Power Supply, what would you suggest for under $150(I could go alittle over if it would be worth it)?
 

this looks like an awefully good deal, and would handle 2 9800GTX's.  
http://www.newegg.com/Product/Prod [...] 6817139006
 
A good 750watt unit should do for you for 2x9800's or 2x GT260s.  Key word, good.  There are cheap off brand PSU's that may have a high wattage rating, but won't handle half of what they claim.  The Corsair has a good rep, and that's as cheap as you'll find a decen psu.  
 
If I were you, I'd upgrade the power supply, and still go with one GT260, and add a second later for SLI.  For well under $300, it's a good deal.
 
Upgrading your ram to 1066 isn't going to improve performance at all.  You'd be just as well to add another 2gb of the same ddr2 800 you have now.  The 1066 ddr2 will only be useful for running FSB higher than 400mhz.  

After reading the specs again on that corsair psu I just recommended to you, I might retract it.  Anyone have SLI experience with the corsair 750?
 
They have a good rep, but it only carries a single 12v rail.  I'd probably go with a model with dual or quad rails.  I'm a fan of the thermaltake toughpower series (they're highly underrated IMO), but it might be a little cost prohibitive for you.  Newegg has a Coolermaster 750W for ~$130 after rebate.  

 
It is a 60 amp 12v Rail. TX750 can spit 720w from that rail alone.  
 
Seriously, what do you prefer? 3 rails with 18 amp each, or 1 with 60 ? I went for that corsair and im extremely pleased.
Whatever card ill put tomorrow, i KNOW it will run. 18/20 amp each gives you a max of 210/240W per rail.
 
Larrabee will consume about 300W. the 4870X2 is near. The GTX280 needs more juice than a flux capacitor. If you are going to slap a SLI, remember, you will have just 240W max to juggle with.
 
I prefer to have 720. Go with the corsair.

if i were u i would go with 2 9800gtx+ assuming ur gonna oc. they reach well passed 840mhz on the core.

2 9800GTX+ or 2 8800GTS

Two 9800GTX+'s are out of OP's budget range and also cost about $30 more than the original 9800GTX's for maybe a 5% increase in performance.  Not really worth it I think

oh im sry wat is ur budget? really only 5% increase even with both of them oc @ 850/1270 from the stock 675/1100 i was thinking more like 10%-12% increase and there pretty cheap 199$.
=]

im lookin to spend around $400 on a video card, and I have $150 for something else whatever that might be... ram, psu, or stickers .
 
Is SLI truely double the performance?
If im looking at benchmark scores for a bunch of cards and I wanted to know how one of those cards would do in SLI mode, could I just double its score to get a someone accurate comparison?

It's almost never twice the performance, but there are rare exceptions.  A more realistic figure would probably depend on your CPU, but I'd say scaling for SLI is somewhere in the ballpark of 1.5x - 1.8x or 50% - 80% more performance.