*censored* do they s*ck
Finally! Somebody with taste!
My old PC had a slower config then yours and I played quite a few decent games. Since you want levels, though, I can recommend Fallout 1 or 2, which should work on that machine. I haven't played them myself, but I've heard good things about them.
They don't really meet your request, but other games that I played on that machine (still play them on this one too) were several Ports of older games- Doom via gzDoom, for example, and Duke Nukem 3d with the "HRP Lite" pack. Quake 1, 2, and 3 and their expansions/mission packs are usually good for quite a few hours of play.