Windows 7 is going to run very poorly on that machine, I would stick with XP. You could get a low profile 8400 GS PCI card but I wouldn't waste any money on that old of a machine. Your machine has a max of 1gb of ram (if you have upgraded it) and Windows 7 is going to want to use most of that by itself even with nothing else running.
Thanks Benski! I was afraid of that! Can't really afford a new computer right now but thru the school I go to, I was able to get a windows 7 install cd. I did upgrade my ram a few months ago but like you said it's only 1 gig.