I have the 8800GTS 640 mb which is kind of old now. I know the 8800 GT is newer so is this a better card? I have seen the GT has better clock speeds but i thought the GTS was higher up than the GT.

Yes, the GT is better than the old GTS (which was either 320 or 640 ram).  But, there is also a new GTS out that has 512 ram, which is better than the GT.
 
GTS 320 or 640
then  
GT
then
GTS 512

 
 What basket case said,
 The main differance is the core its a G80 for the older GTS cards and a G92 for the new GT and GTS .
    Mactronix

The GT only has 512MB and the GTS has 640MB. I thought this was the most important thing so the extra MB would make up for the slower clock speeds.

performance is:
 
GTS 320/640
GT
GTS 512
GTX
Ultra

The performance between the GTS 640 and the GT is so marginal I really hope you aren't looking to upgrade to that.  Unless you are looking to SLI the GT with another then the fact that it consumes less power might be a compelling reason to sell it.  If you are replacing your card with a GT straight up, you won't be wowwed.

 
This is true.  Not much of a difference between them and it isn't worth the hassle.

Yeah, i wasn't even thinking... I just got caught up on the "which one is better..." aspect of the question.  If the OP has a 8800GTS 640, just stick with it.  You will be wasting money upgrading to a GT (or a new GTS).
 
It would be like (in the old days) upgrading from a ATI 9800 Pro to a ATI 9800 XT... not much different.

 
The GTS 512 does better than the GTX in some cases, id say the two are equals.

 
And this is not true at all. Memory size, although it does affect performance, really only matters depending on the resolution you play at.  
 
So if you play at a really small resolution, like 800x600, it wouldn't really matter if you had 256MB or 512MB, because the card wouldn't use all of that memory. But as the resolution increases, memory does become more relevant.
 
And this is only for today's games. I'm sure in ten years or whatever you're going to need like at least 1 gig of onboard memory just to play at 800x600, because the textures and other aspects of the game will be so complex.
 
So there are a few things that can affect the amount of video memory you need, but generally in the short term, you just need to consider the resolution you play at. I would suggest 512MB or more if you play at 1680x1050 or 1600x1200 or higher.  And I think by now with today's games, and with prices so cheap, I wouldn't suggest anyone get a card with 256MB unless they are playing at less than 1280x1024.
 
So your 640MB should be fine for quite some time. And I do realize that isn't what you asked at all, but sometimes I go off on tangents....
 
And yes, the 8800gt is faster, but not by much, and as others have said, it would be better just to keep what you have and wait until something comes out that will truly be a nice upgrade.

I was actually contemplating something similar to this as well.  I currently have an 8800GTS 640.  I was fine when I was playing at 1280x1024, but I have since changed to playing at 1920x1080, which in some games starts pushing the limits of what it can handle.  Crysis is where it shows the most(duh!) but I do see it in other games, Supcom, Gears of War, and a few others.  I was trying to decide between a second 8800GTS, selling my current one(already have an offer for $175) and getting a pair of 8800GT's, a pair of 8800GTS(512MB), or, if I really felt like breaking the bank, a pair of 8800GTS 1 GB(depends on how my stocks do).  I do like to OC, and I like the numbers I've been hearing for the new 8800GTS.   I'm pretty certain that everything except the 8800GTs would need a bigger PSU, and I'd probably wind up getting a bigger one regardless of what I decide.  I haven't seen too many places include the old 8800GTS in the comparison to the new one at the resolutions I'm using, and Tom's hasn't put the 8800GTS in SLI in there VGA charts yet.

 
You might just want to sell your gts and get a gtx. The extra memory would help at 1920X1080(I think thats one of the things you were talking about), and it does have extra performance. I think that's probably one of your best options if you think you need more video memory, because sli won't give you more, especially if you sli 2 of the videocards with 512.
 
But if you don't think it's a memory problem (I wasn't really sure if that's what you meant or not), then it would probably be best to sell what you have and sli two 8800gt's.
 
I would actually recommend this, because two 8800gt's would cost about the same as one 8800gtx.
 
Again, the 8800gts 640 is still a good card, but if you feel it isn'e enough for you, and you have the money to do it, then go ahead and get new video cards.

 
Yeah I thought so too, strange how each month on the 'Best Video Cards for your money' the 8800GTX is placed one tier higher than the 8800GTS 512

Id sell the GTS for the 175 and get the new 9800GTX http://www.zipzoomfly.com/jsp/Prod [...] e=10008128 as itll be better than an Ultra for $175 and handle most games (evcluding crysis) at your res