Well since UT2004 is pretty forgiving when it comes to video performance I would probably suggest a Radeon 9600 AGP video card. I am assuming here that you have an AGP slot.
A good card to get would be the Sapphire Radeon 9600, with 256mb ddr. Its memory bus is 128-bit so its safe. It can be had for about 95 dollars or so. Also Rosewill makes a cheap 9600, but I don't know much about that company.
My son has the same card in his computer and it plays UT2004 very well.
Just make sure you stay away from the cards that have 64-bit memory buses slows them down alot.
edited for spelling issues.