GF2MX to a GF4TI will completely blow you away! Definetly go for it. The performance will be decent, and you could probably play UT2k3 at 640x480 resolution, but that's because of your CPU and memory. But I think it would work fine.
If you decide you wanna upgrade the system too, but dont wanna spend too much money, get an ECS K7S5A + AthlonXP 1600 for about $110us at http://www.newegg.com
. It'll take the SDRAM you have now, but also has DDR slots when you're ready to upgrade later.