You could always try overclocking your 7900GS even more, I had mine at about 590Mhz core and 890Mhz memory without any additional cooling. I then bought a Zalman VF-900 and could get 600 core and 900 memory, and finally volt-modded the core(quite simple actually, there are guides online) to get 690Mhz Core and 900Mhz memory(didn't volt mod this, so no increase). You might find you can squeeze a bit more mileage out of it that way so you can save up for a true upgrade like a 3850, 3870 or 8800GT.