Sounds like my previous computer which was a tower (got a Dell Latitude D600 laptop now), also P4 2.4Ghz and also 40GB hard drive, it was a Gigabyte mainboard with some crappy onboard souncard and just a GForce 440 MX or something like that, in a card slot, then I replaced it with GForce FX 5700 with 256 MB RAM which was a lot more and better.
Glad you got your own computer now, it's always good thing to have a little bit more of own freedom. If I was you, though, I would upgrade the computer to at least 1GB RAM (that's a much as my had in the end, too), 512 is just too little on a long run and 1GB is the today's standard with more being a good choice.
Anyway, enjoy
k: