well it depends on what you are going to use the PC for.
if you are a gamer then you will probably want to get something pretty decent, entry level i would say for a system like your for a game would be a radeon 8500, however i have had problems with a radeon in a KT266a motherboard, but i highly doubt you would.
this card will do pretty much anything you want it it, also has great 2d IQ (image Quality) and you should be able to pick one up for under 100 dollars by now.
if you want something a little more juicy, in case you are a gamer who is playing the newer games like MOH:AA or SOF2 or maybe you are waiting for Unreal Tournament 2003 to come out. then i would suggest getting something like a geforce 4 TI 4200, 4400, 4600 either one should be great for these games, i am optimistic about UT2003 in that you wont need a radeon 9700 to play the game smoothly.
although if you do get a radeon 9700 (if you can find) then you will be getting an awesome card, there was a thread on this forum showing UT2003 running on a radeon 9700 and it was simply stunning.
if you are just gonna be using the PC for office use, or maybe just general surfing the internet then you dont need to spend a lot, you could get a geforce 2 mx for about 30-40 dollars and if you wanted to play the occasional game you could.
ATI cards seem to have sharper 2D IQ over nvidia cards in my opinion and apprently matrox have some great 2D image quality cards as well, although i have yet to try one out.
if i were you i would probably get a radeon 9700, mainly because i am a bit of a hardcore gamer, but if you are not so hardcore, or if you have a budget in mind then the radeon 8500 would be an excellent card nonetheless