Geez, you have an old system. You're looking at a system that's on the brink of obsolescence. it can run Fallout 3 most likely because of XP being that much better than Vista / 7 with system resources. also its hte game developers that say the system requirements, its always set so that 100% guaranteed that any system that meets those specs will be able to run the game. your system may not meet those req's, so that that means its a hit or miss getting the game to run. In other worlds, your system is not supported, if it doesn't run on your computer its not the game maker's problem.
you can upgrade that CPU if its lga 478 or 775, socket 478 being harder to find. check out craigslist or eBay, more or less someone will have a spare compatible cpu, it'll be cheap and its really a drop-in type upgrade.
I've already done similar with my old computer (a 2002 Dell Dimension 4500), swapped the orginal socket 478 MoBo for a used Intel LGA 775 MoBo, and dropped in a Intel D 805 cpu i got off craigslist. Swapped the orginal PSU for a slightly more beefier one also made by Dell. whole upgrade was only about 40 bucks. actually the CPU & fan was 40 bucks, I got the motherboard and power supply for free because a local school was tossing out their systems and I asked about it, got a free computer.