I have a tight budget-minded-bias so here goes:
Your computer seems to be fast enough to be able to play those games fairly fine at your 12x10 resolution, so I would say to sit on your current setup, and wait for G71 or ATi's next offering. The fact that you have an open pci-e slot on your mobo is what is tempting you to drop $300 dollars on another card where, in your situation, you would probably only see a nominal increase in visual benefits, as compared to what the next generation card can provide you.
On the other hand, you can sit on your hands forever in this game waiting for the next "thing". Take me for example, I just popped in another 512 ram to give me a gig, on an XP 2000, with a ti4200, and I am eternally waiting.
