Hello,
 
     I have another rig with this card in it: "EVGA 640-P2-N821-AR GeForce 8800GTS 640MB 320-bit"
 
Newegg says this card is deactivated. Is there another 8800GTS that I can SLI with it, or does it have to be EXACTLY the same card? I'm having trouble finding an existing GTS card that has matching specs.

You want one with the same chip (8800GTS 640) at least.  I thought that if one card was clocked higher it would just downclock it, but I could be thinking of crossfire.  Are there any 8800GTSes left?

wow it's not available anymore? that must have happened as recently as today or yesterday.  im sure other sites are still selling it.  I'm almost certain you could probably find it in a store somewhere still.

just get this one your cards will run st the same speed in sli. to ansewer your question you need the same gpu so you could use the 320 mb version but i wouldn't because it will disable the 640mb to 320 on your card you have. http://www.newegg.com/Product/Prod [...] 6814130317

I just looked in to this last weekend, as I'd like to do the same thing, with the same expired card.  My reply from EVGA was that you can buy any other nVidia card and configure them in SLI mode and they will work.  They will, however both run at the slowest speed of the 2.
 
hope this answers your question.

It does. Thank you for your responses.

http://www.newegg.com/Product/Prod [...] 6814130327

The problem with that though is that the card isn't really worth $250... at least to a new buyer where better cards can be had for less.  But in the OP's case the situation is a bit more in the gray area since I don't think you could get the same performance (2x GTS640) for less in any other way.  A single new $250 card won't outperform the SLI, and an SLI that would outperform 2x GTS640 would be far more expensive.
 
-mcg

It's only $220 after MIR  

I had considered upgrading to a newer single card, but I read that the performance increase of those cards over something like an SLI'd 8800GTS 640 that I have is not very significant.  
 
I could also wait and see what the new Nvidia card (GT200?) has to offer.

Well if you wait u will always be waiting, because something better will always be coming out. Thats what sucks about the 8800 series it was a large jump from the 7900 series and the 9800 series is not a large jump from the 8800 series; well, when compared to sli.