I had a X1950GT up until recently and IIRC it's somewhere in the range of the 7900GS. It definitely struggles at 1280x1024 on new games like Crysis (though that is also attributable to the single core AMD64) but Left4Dead runs absolutely fine at max settings. Source engine = rediculously low requirements.
I had
AMD 64 Single core @ 2.4ghz
2gb DDR400ram
X1950GT oc'ed to X1950pro levels