768mb RAM? You really need 2gb more of RAM, it's only $30 dollars/£25 at the most
Anyways, Papers Please has been shown a lot lately, you've probably heard of it, it's easy to run and very fun. I think it's £6.
The old Call of Duties, which haven't got multiplayer but are generally pretty fun, Call of Duty 2 will run ok on your System
The old FarCry is a possibility
Bad games with bad graphics is probably all you're going to get, that system was outdated years ago