The 8800GS is the one to keep IMO. But I'd suggest you test them for yourself and see what is best in your games at the settings your desire to play at.
8800GS clock speeds vary by a huge degree. The core can be anywhere from 550MHz to 680MHz out of the box, the mem can be anywhere from 1.4GHz to 1.9GHz (effective DDR). An X1950XTX could probably take a few wins against a low clocked 8800GS. A high clocked 8800GS should pull ahead even further though, even topping a 9600GT at times (especially no fsaa).
By get rid of the card... are you trying to sell it? Bet you get more money out of the 8800. Simple reason for that is the 8800 is likely to out perform the 1950. If you really are not interested in DX 10... and either of these cards will allow you to play what you want to play (at playable frame rates), then you should sell what yeilds the most money. That is likely to be the 8800. Still, if I were going to keep one, it would be the 8800.
One thing that helps a lot in figuring out which card is fastest is downloading FRAPS and running it while playing a few games. Do that with one card, write down the numbers, and then repeat with the other. As Pauldh said, a 8800 GS can vary a lot, from being the slowest to being the faster of the two. The only way to know for sure is by testing with FRAPS.
The 8800GS is the XFX Alpha Dog Edition So the Memory is 1600mhz and the core clock is 680Mhz so this card would out perform the x1950xtx? I cant be arsed to test the 2 cards together (too much faffing around)
So in conclusion...
The system will be running XP MCE
2 Gigs of Mem
and 17" TFT @ 1440x900