I've found Quake 3 Arena to be a great game to play when you've got a handful of older low powered computers to play on. My friends and I occasionally play it on our school's 3 year old library computers at lunch, and of course those computers were not made for gaming at all, yet it runs at a smooth 91 FPS. It doesn't take much to run it seeing as it was released in 1999.
Speaking of 1999, Unreal Tournament 1999 is even better performance-wise than Unreal Tournament 2004. Counter-Strike 1.6 is likewise quite easy to run, and possibly even Counter-Strike: Source, if you play it on minimum settings... the Source engine is very well optimized. If CS:S works, that opens up Half-Life 2: Deathmatch as well. Another option would be some of the older Doom games (not Doom 3).
Honorable mention, though it is not an FPS: Warcraft 3 runs very well on older computers, if you are interested. It's an RTS, but there are quite an assortment of custom maps for it, many of which are in TD or RPG style.
All the games I mentioned, besides perhaps the ones that run on the Source engine, should run very smoothly on the computer you listed.
