If you have agp, then you cant go with the X600 series as they are only PCI-E. The 9800SE is very very stripped down. I think even an 9600 XT will outperform it. You may want to refer some of the other more informative articles around here for elaboration. If you are in want of an ATI card, and your price range is in the 100s. The 9600 pro, 9600XT will be for you. For high $100s and very low $200s range. THe 9800pro is your pick. If you can afford 20$ more, the 6600GT AGP version from nVidea will be a huge upgrade.
In other words:
100-120$ = 9600pro
120$-160$ = 9600XT/used 9700pro preferably
190$-210$ = 9800pro
$220-$240$ = 6600GT AGP from nVidea.