You can get a pretty good AGP card for $150, but the problem is that AGP isn't really the standard for video cards anymore. Nowadays, the vast majority of cards are PCI-E. If he wants a machine that will play the newest games, and do so for the next few years, that's the path he needs to go down. Unfortunately, the HP model you mention doesn't appear to have any PCI-e slots, so you're looking at a new mobo at minimum to accomplish that (and possibly a better power supply, etc.). With that many upgrades, you're dealing with virtually a new PC.

Sorry I don't have better news for you. I found all this out a few years ago when Civ 4 came out and my old rig couldn't play it.

I have this one. Depending on which one you get and where, you will exceed the $150 budget. It is, however, AGP and will work fine with that setup.

I agree that the "right" answer is PCI-E and a new mainboard, but in my case, budget-wise I couldn't afford it. The 7800 GS is the next best option and is considered one of the best (if not THE best) AGP card available. My machine is similar to your cousins, only with a 2.08 AMD processor.

This machine runs many, not all, current games at decent resolutions - I have a 22" monitor and run Age of Empires at 1680x1050 without breaking a sweat. Half Life 2 is no problem either. If I get into a lot of textures or really CPU-intensive stuff (the water in 1701AD can completely kill this guy), the system dies pretty quickly though. Which is why I really suggest that you get a new, really horsey power supply for the rig. This video card can single-handedly shut down the PC based on current draw alone (it has two power sources, the PS directly and the mainboard AGP slot).