Upgrade your RAM, XP will not be happy with 256Mb, let alone games.
If you're into games, consider either a few upgrades or an entirely new PC.
I realise that may not be an option right now, but no modern games will work on that setup.
Aside from that, I highly recommend the Command&Conquer series (not the latest one though, it's too demanding).
Also, how about the old Unreal and Unreal Tournament games? (Not the 04 version, too demanding)
They were pretty good.
Halo PC might be worth a look too.
I realise most of those are shooters, I play FPS games mostly and no RPGs.
Just a few suggestions for you anyway.