As you have discovered, your card is evil. I'm building an entire new system next month and have been researching parts for three months now. I'm a NVIDIA fan so take it with a grain of salt.
The 7600 GT is THE sweet spot in the best bang for the buck catagory. I'm going to assume your interface is PCIE and your old system was AGP? If I'm wrong and both your computers have the same graphics interface, put your 6800 in your new rig and save your gold for a more expensive card.
The 7900GT is a great performer. Stay away from the overclocked cards as there is a tenency for some of them to artifact. The stock speeds on them are plenty fast.
Do you need SLI (two cards) or does it just sound cool? I play BF2 with some buddies and one has a 7800GTX, Athlon64 3200 processor. The other has a sli setup (2)7900GT's, Athlon64 4000 processor. Aside from those specs their systems are identical. Same mobo, same ram, etc. The first guy pulls 90-99 fps with all settings on default high in heavy combat. The second guy pulls 99 fps with all settings on default high. The game locks the frames at 99.
Are the extra 9 frames per second worth the additional $300 he spent on a second video card to run SLI? Nobody I know can see the difference between 90 and 99 frames per second.
It is to him and he's happy. Did he future proof his rig for more demanding games down the road? Maybe, maybe not. DX10 could shake that theory up a bit.
Decide for yourself.
*Edit- Check your powersupply specs, note the amps on the 12v rails. Idealy you want it to say 18amps or greater. I'm also going to hope you have a power supply that is rated at 350w or greater. I'm too lazy to look up that cards power requirements but if it matches both those, you are good to go on the 7600GT.
If you need to get another power supply to get another vid card use the above "stats" as a guide line and look for a 400w or greater one. You'll want to spend near $100 on one or risk it having some personality...
Later