I dissagree. A 6600GT scores a measly 7000-10000 pts on a PIII machine depending on motherboard chipset and cpu used (coppermine or fast tualatin). It's wasted on a pIII machine. On a 3.4GHz P4 a good 6600GT will rock a solid 18000-20000 points. A 9600XT would be more appropriate and easier to get, but a 9800xt is again wasted on such a machine. You are better off with a 128 bit fx5600.
My 9800 PRO does 20000-22000 pts on a fast p4 or A64 rig, and barly scratches 10000pts in my 1333MHz Tualeron.
What I'm trying to say there is no point in wasting time and money on a fast modern graphics card for this machine. Not to mention some newer cards (including the 6600GT) don't support 3.3v agp signaling most of the time. I seriously doubt the 7600gt will work in a PIII board in the first place (of course I could be wrong). As far as I know both cards are keyed for agp 8x motherboards only, so it won't physically fit into an older 440bx board with AGP2x. Other socket 370 / slot 1 chipsets do not support the 1.5v / 0.8v agp signalling required to run modern agp cards but the card will phisically fit in the AGP slot. In that situation you risk ruining both your motherboard and gpu.
Just stick with what works - if you play DX8.1 games get a Geforce 3 or 4 Titanium (NOT THE MX IT SUCKS) or a radeon 8500, and for DX9 games get an FX5xxx or radeon 9xxx.
The GF4 Ti is FAST - 14000pts in 3dmark 01 on an athlon64 - it can play black and white at 1920x1080 in winXP with a minimum fps of 20 and maximum of 55.
Then again, best to do whatever makes you happy! It's your build, put whatever makes you happy in it!