I was 14 when I got my first gaming PC, so I didn't even have a job. I saved up about $600 dollars mowing lawns, selling old software on eBay, and babysitting (NEVER AGAIN!!!). It took me about 5 4 or 5 months or so, and I had to persuade my dad to loan (yes loan) me about $110, but I went to a shop and had them build my PC. I've upgraded sound, speakers, vid card, and OS since then, butI still was able to get a good deal. At least you've got a job, I didn't have crap. I make $7.75 now, so in a couple of years I'll buy a pretty nice PC when I head for college.
EDIT: Oh, and don't upgrade that P2. You'd have to get a new mobo for the new CPU, so of course new memory, new grafix card, then sound, etc. You'll have bought yourself a completely new PC. At the best you can salvage the CD-ROM and Floppy drive from it, and maybe the hard drive, which you can set up as a Slave. Not even the case will work; it's not ATX.