ok this is with regard to an OP's dilemma...i found the problem rather interesting.
 
X has an 8800GTS 640mb. X wants more juice for gaming. His budget is not more than $200-$220. What should X do?? Should he get another 8800gts so he could SLI it up? Is that an improvement? Is the improvement better than one G92 8800GT or GTS? If yes, are there any benchmarks to compare one G92 8800GT/GTS with the 8800GTS 640........... or is there some OTHER solution?
 
 
 

Well 8800GTS 640MB SLI is certainly better than an 8800GTS 512MB.
You dont really need any benchmarks for that.It's pretty straightforward.

^ ..straightforward..how? Have you tried SLIing the GTS640? Have you compared it with the G92? Theres no concept of straightforward when it comes to SLI. Thats exactly why this thread is posted.

Obviously, it depends a bit on the game.  Not every game takes full advantage of SLI.  But, most of them do see some gains, and some of those see significant gains.  While a single GTS 640 is slower than a GT 512, it isn't remarkably slower.  When you SLI it, though, it should perform faster in just about every situation, assuming you are playing a game that can use SLI (which is just about every game, now days).  So, if someone had one GTS 640, and they wanted faster performance, getting another will be faster than getting a GT 512.  Now, the other factor is Resolution.  As we all know, the benefits of SLI become apparent when running higher resolutions.  So, lets go back to the scenario.  If this person only ran games at 1024x768 with their GTS 640, they are not going to see a ton of gains with a second one, since most of the games they play probably run pretty high FPS already.  Adding another one will give better performance, but they already had it.  But, if they play games at 1650x1000 or higher, they would see some huge gains with the second card added.  They would also see gains jumping to a single GT 512, but at the higher resolutions, they will still run into the same problems as having a single card.  
 

Well you are sounding as if "G92" is some sort of a monster or something.
8800GT is 10% better than the 640GTS and the new GTS is about 20-25%  faster.You think SLI will bring only 25% increase?
Well maybe the new GTS will be able to keep up at lower res but certainly not at higher res and\or high AA.

Another issue is power.  Make sure his PSU has the +12V amps for the second card.

I have gts 640's in sli. I can tell you that it allowed me to raise crysis from medium to high on a 22 inch monitor at its native resolution. To be honest, I haven't seen the gain I had hoped for compared to the money spent.

I think that answers the question,
 
I could run Crysis will all high with 1680x1050 res. with a single 8800GT. Givin no AA but, that doesnt seam to make much of a difference anyway.

I was facing a similar dilemma and decided to wait a bit.  
 
What I did to compare performance, I did right here at Tomshardware.
 
Open one window with single GPU, select the 8800 GTS G92, and in another window open up the dual GPU and select 8800 GTS 640.  
 
That should give you an idea of what performance differences you'll get.
 
My solution presented itself, since my father-in-law wanted a new compueter, so I'm exchanging out the 640 (putting it in his computer) for the new GTS and will SLI later.

 
When did you buy them?  Now, lets say someone has had one for awhile and wants to add the second one now, it can be a good upgrade, because they are much cheaper now than a few months ago.  You yourself said you could up from medium to high in Crysis on a 22" monitors native resolutions (which I assume is 1650x1000 or higher).  That is actually a pretty strong achievement, as Crysis is a bitch of a program to run.  
 
No matter how you swing it, they are going to get more performance out of two SLI 640's than a single GT 512.  Will it always be a lot more...?  Not neccesarily, but it will be better.  

8800GT barely beats the 640GTS in crysis.So it's obvious that there is some other variable coming in here like other components,the amount of OC on the GPU and one's own "standards" for what is playable and what is not.And some people dont like to play without AA and AF.
http://www.techspot.com/news/27877 [...] y-out.html
http://www.techspot.com/review/79- [...] page2.html
 

 
I have the same setup. Bought the first one for $400 and the second for $250. for me it was very much worth it. I saw the best SLI scaling in ETQW and COD4. Marginal improvement for Crysis, but I play it in DX9 hacked with some Very High settings at 1920x1200.

^..Ok now....SLI isnt perfect..yet...SLI performance varies for different games and different GPUs.. even if card A is marginally faster than card B, it doesn't necessarily mean that the card B SLI would be faster than the single card A ....................thats exactly why this post becomes interesting...what should the OP do? Use 'em grey cells people!
 

 
Um I was just relating my personal experience with the setup in question.

ok wait...by "^" i meant Thinker_145

 
 
Ah my bad, it did seem a bit out of place.

Benchmarks..........anyone!!!??  
 
Solution.......anyone?????

 
 
I agree, my system was OC'ed quite a bit. But perhaps that gives him another option. Instead of spending extra cash on another VGA, to get marginal performance increases, he could OC the entire system and get the same marginal or better performance for spending no cash. This is assuming he has decent cooler's on his equipment.

 
I bought them approx. one year apart. Can't remember the exact price of the second one however it was significantly cheaper. Res is 1650x1000 you are correct. I was pleased with the jump in Crysis actually, I had heard through the grapevine that a 512 gt could do what my sli was doing though so that was a bit of a bummer. That is all hearsay as far as actual performance so take it with a grain of salt. What I can say is...Crysis on high runs smooth with no AA on a pair of gts640's in sli... end of too much information!

Ok the dudes using SLI  640mb GTS....can anyone of you pls use fraps and put up a few benchmarks? For three to four games...(Crysis included)