2x8800GT vs 5850

Off the top of my head, the 5850 would be about 50% faster,or more, then the 8800GT's in SLI. The 5850 is the clear winner here, imo.

Not quite right.
SLIed 8800GT's will perform fairly close to about 15% slower than a single 5850.
Based on SLIed 8800GT's performing about the same as a 9800GX2, the GX2 performing about the same as a GTX285 and the 5850 matching to outpacing by 10ish% the GTX285.

Personally, I would get the 5850 and ebay the 8800GT.
Beyond that, you will be quite CPU limited.
I would recommend saving for an inexpensive AM3 platform upgrade.

Oh yes, about CPU, will it bottleneck the strong videocard(s) ? And what is a nice CPU when we're talking about a HD5850/2x8800GTs?

You could probably get an Athlon 2.

Yes, your current CPU will bottleneck you planned GPU upgrade.
As to which is best, that really depends on your budget and what else you use your machine for.
Check out This Article for some good recommendations or post back more information for anything more specific.

the gx2 is only slightly powerful than a 260 in terms of maximum frame rate but it gets slaughtered by the gtx 260 when it comes to minimum frame rate.it can maintain better minimum frame rate as it is not a "SLI on a stick" card like the gx2.the gtx 260,280=8800GTS in sli so the single 5850 will pound the 8800gt sli as also will the gtx 260

AS I posted in the double post:

A 8800Gt will cost you $136? Man, can't you get one on eBay for like $50? $136 will get you a single 5770 512MB.

Here is a link to a Tom's chart that compares both setups. The (2) 8800 GT's in SLI gives a single 5850 a good run. In most cases it comes up a little short or slightly behind, in a few it beats it out slightly.

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If it's a $50 upgrade, I would get a 2nd 8800GT if you want a cheap good upgrade. If you are going to fork over $136 I would sell your 8800 and just get the 5850.

^I guess it doesn't like a link full of []'s just copy and past the http:// part

As mentioned your CPU will/could bottleneck your video performance in some games. It all depends on the title. Some games like fast or quad core CPU's. Battlefield: Bad Company is one of them. I would check your motherboard model on the manufacturer's website. IF its an AM2+ you can support some of the newer chips with a BIOs update.

its an AM2 m/b and it wont support anything else than an AM2 socket.

BC2 is exactly the reason why I'm doing the upgrade - it lags slightly sometimes, are you saying its CPU dependand then? That's sad sad news indeed...

BC2 scales well with most hardware regardless. Try your hand at OCing your CPU, it would give you a slight boost. If you aren't comfortable with it, don't worry about it. What are your settings? Screen resolution? It hits your CPU because of Physic in the game. Dual core CPU's tend to run around 90%+ utilization when gaming. It recommends a quad core, however, a dual core works fine. The faster you get your CPU the better it will run.
As far as your graphics card, if you turn off HBAO it will help alot. HBAO is a lighting setting it gives the really awesome shadows in the game. However, it doesn't distract from the actual detail in the game. Also, turn down AA and AF. THose 3 settings put the most hurt on the graphics card. Try it out and see. A 2nd 8800GT will help alot or a 5850. Ultimately your CPU will be a limiter in your settings. However, Bad Company 2 scales REALLY well and looks good even on medium settings, with AA x1,AFx1, and HBAO off.

My system is an i7 920 @ 4GHz and a 5870 stock. I can max it out with DX11 and all settings turned up at 1680x1050. HOWEVER, my 5870 had to go back for an RMA because it would artifact once the card hit a certain temp. I am currently borrowing an 8800GTS/512 card, which is all but an 8800GT with slightly higher clocks.
With the 8800GTS I had to turn down AA and AF to x1 and HBAO OFF. Other than that everything is on HIGH and Vsync is ON. I get a constant 60FPS at 1680x1050. I attribute this to the fact that my CPU takes card of the Physics and the game well. I have to say that even with this temporary solution it still looks GOOD.

yeah the game looks awsome even with med settings and a single 8800GT, it just lags slightly more with my config than say MW2...

That is because MW2 is DX9 and an old engine. BC2 is DX9,10, or 11 and has an awesome physics engine.

one last question: lets say I decide to buy a second 8800GT and leave the rest of my config as it is, considering my CPU will bottleneck the 8800GT SLI, how big of a performance increase can I expect from this upgrade?

No idea. It will depend on the game really. Most games now really take advantage of SLI or Crossfire and see over 50% performance increases, some upwards of 90%.