Doom 3 looks amazing for how well it runs on older hardware. Its one of the most fun First person shooters, that might be less demanding on hardware. Its optimized about as well as a product can be, John carmack is kinda elite.
Try doom 3. If that wont run, youll have a hard time finding great FPS'ers.
also Ran impressively well on my 9800 nvidia system. But I am not sure if your hardware can keep up with the old old 9800 Video cards or not.
It just might though, the cards showing its age fast.
Battlefield 1942 should be very easy for older systems to handle, there may or may not be alot of players left playing that online, but there sure may be.
Same with Steams Counter strike.
If those wont run, those are some of the easiest to run that i know of,
unless you go back to wolfenstien 3d (Part 1 and 2) Spear of destiny.
doom 1 or doom 2 lol
I assume you have the Intel GMA 4500 graphics core... Some games that you can play...
Below are links to Youtube videos of games played with the GMA 4500. Read the poster's comments for possible instructions on tweaking the game to get it up and running; like for Fallout 3. Also read out frame rates as well since many people generally do not like to play at less than 24FPS, but other don't mind...
Doom 3 - Video shows it running on an older Intel Extreme Graphics 2 core. Since it is the same game engine used in Quake 4 the GMA 4500 should be able to run the game. http://www.youtube.com/watch?v=P4T_LZ4R3n0