Hi Cyber_hawke,
Welcome to PCHF and sorry for the slow response.
As long as you're willing to keep your old accessories (mouse, keyboard, monitor, case, computer chair etc.) You only really need to purchace (for a full upgrade) a
CPU and motherboard,a new Power Supply, compatible
ram and more than likely a new video card. Hopefully you have a decent sound card and a good hard drive in your current machine and you can stick with your old cd/dvd rom/writers and whatnot. Set aside some coin for a new XP license too. other than the few parts just mentioned all the rest is fluff, except for fans you may want to get fans but those don't cost too much.
It all really depends on what you wat the primary use for your computer to be. If you want to play all the new games you will probably want to go all out on
CPU and motherboard that way you will have room for expansion as you need more guts. If you're just doing office stuff or schoolwork or whatever you don't need the most expensive everything, in fact for those purposes a 64 bit or dual core rocessor may be overkill, but m'eh it's still fun. Always get good quality
ram and' i'd get the fastest
ram that your motherboard will support, if you're planning on going with dual channel
ram, start with two smaller sticks if you want to cut initial costs.
Hope that answered at least some of your questions, feel free to ask about anything left unclear (as i'm good at leaving things kinda muddy

)