Okay, here's the deal, I currently have one 8800 GTS 640 SSC I picked up last January.  It works well for all of my games (Company of Heroes, Supreme Commander, COD4, Oblivion, etc) at 1440x900 on Very High settings.  
 
I want to be able to game on my LCD TV at 1920x1080, should I just pick up another 8800 GTS 640 or should I spend some time trying to sell the 8800 GTS 640 and then pick up two 8800 GTS 512s?  Will the difference really be that much between the two SLI configurations at high resolutions (not including Crysis)?
 
This is the GTS 640 model I purchased last year: http://www.newegg.com/product/prod [...] 6814130080
 
This is the GTS 640 model I was thinking about getting: http://www.newegg.com/Product/Prod [...] 6814130317
 
They are both G80, but my discontinued model only has 96 stream processors while the new one has 112.  Will they still be SLI compatible if I go that route?  Either way they'll be paired with a Q6600 at 3GHz on an ASUS P5N32-E SLI 680i board.
 
Thanks in advance for the advice!
 

Won't be SLI compatible if they have different architecture. But if they're 640MB cards, they should both have 96 stream processors. all 640MB cards have 96 stream processors, if it says it has more it's a typo. only the 512MB cards have [EDIT]128 stream processors.
 
Your 640MB card will go a good job in 1920x1200 in all of the games you listed, but you'll have to lower the detail a smidge. I don't think I'
d recommend buying another card to SLI it, it'd make more sense to sell your old one and put the cash toward a new 8800 GTS 512MB card.  

 
I'm sorry, but that's incorrect. The 8800GTS 512 has 128 stream processors. The 8800GTS 320/640 SSC edition have 112 stream processors. I don't know if they'll SLI together or not, I'd be willing to bet they will though, but only let both cards use 96 stream processors.  
 
I had the 640 model, and upgraded to the 512 model, and kind of wish I hadn't. There IS a difference, but not worth the upgrade, IMO. I think you'll probably be ok with your card now at 1920x1080. I played my GTS 640 at 1680x1050, and it did very well. If you find yourself having to turn things down, you could always try OC'ing it.

Both cards are compatible but the newer card has a faster memory speed and will reduce it's speed to 1800 to match the slower card unless you overclock.  
 
I think your best option would be to sell the card you have and pick up either 2 GT's or 2 of the new GTS 512's.  Either of those setups would be faster than an Ultra and if you decide on 2 GT's you would end up paying a total of around $200.  I sold my GTX a while ago and used the money to buy 2 GT's and I got a nice performance gain for the $20 it cost me after selling the GTX.

Why don't you try your current card on the LCD and see how it does first?

Thanks everyone for your help.  So I think I've decided to either save the cash I planned on spending and stick with what I currently have and maybe upgrade later when the 9800s come out, or buy that Philip's amBX gamer kit and pray it doesn't suck!  

 
Until I saw these responses, I thought I would have no real hope of playing at decent frame rates on the LCD.  Didn't want to waste the time lugging my heavy antec P180 downstairs if it wouldn't work.  Checked the VGA charts of course, but no overclocked 640GTS is there, just the 320, and I didn't want to make any assumptions about performance.  I checked several other sites but I always feel more comfortable with the charts here.

 
Oops about the 512, that was a typo - I meant 128.
 
However, didn't know about the 112 processor SSC editions of the 8800 GTS. Looks like an EVGA-specific tweak, I don't think it's an official Nvidia model of 8800 GTS.

 
No problem  
Yeah, I believe EVGA is the only one to do that.

^Agreed. Evga cards FTW!

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My 1st PC(2003) OCed/Built
E2180 @3.2Ghz + P35DS3L +8400GS (700/475 OC)  

Just hang in there for the 9800 series, I have a GTS 640 since last may and run all my games at 1920 x 1200 all high and do very well since. But Im waiting for the 9800 GX2 to replace mine this spring.  
 
 

^^ crazy setup.

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