Try the "oldies but goodies" kind of videogame:
If I remember correctly, I think your computer can handle Halo 1 for the PC...my old pc was 256RAM, AMD733, NVIDIA 5800 128Mb and I can play it at a good speed on lower graphical settings.
Here are some that might push the limit but are good:
1. Battlefield 1942 (love this game, really scales well on computers)
2. Call of Duty 1 and expansions, and MAYBE CoD2
3. Starwars Republic Commando (this game looks GREAT but is so efficient even on slower pcs)
4. Serious Sam games (yeah, they are just pure fun, Im sure your pc can handle it)
5. MAYBE Battlefield 2 (again, this game looks GREAT even on slower pcs
6. System Shock(s) =P can't play Bioshock might as well go backwards
7. Medal of Honor allied assault (Im sure your pc can handle this one...another highly rated world war 2 shooter...one of the classics)
....and tons more...google them
Since you make a comment about runescape, I'll assume you like RPG games, here are a few to try:
1. Diablo 1 and expansion (ANY computer these days can easily handle it. The 2D graphics in this game is outstanding, not to mention the hours of hack n slash lol)
2. Dungeon Siege 1 and Legend of Aranna (I love dungeon siege games, especially the older ones. They should run good on your pc)
3. Marvel Ultimate Alliance (one of the best superhero games I've ever played. This might be a stretch for your pc, even on the lower settings)
If you want mmorpgs...Dont get sucked in
1. Definitely runescape.
2. Last Chaos isnt bad, and its free with decent graphics
3. Eudemon (looks good, its a mediocre game)
If you have a decent interweb connection that is.
have fun gaming, don't play too much...
Derg