well if the pc's gotta be a hand me down when you upgrade, then maybe its worth it to give it one last heave ho in the graphics department. your best bet is to buy used...you can get some good deals
ive seen used 9500's, 9500pro, 9700, 9700pros go for less than £50 on place like ebay. (9500pro i was looking at went for a mere £26) and you know, they are good cards. my buddy still got his 9700pro, plays HL2 just fine, doom3, quake 4, BF2 all at decent settings too.
my brother still plays on my old 9500pro, and hes a regular CS:S, guild wars player. BF2 punishes it abit, but i think thats just because of the rest of the machine not being up to par.
since its AGP, i probably would try go as cheap as possible, while getting a decent upgrade. i think a used 9700pro will be a good choice, you may even get hold of a 9800pro....who knows!
6600GT or something with similar power will be a heft upgrade, should prolong the computers life somewhat, but id bet your gonna have to shell out the 100 bux for that