First, get a DVD drive, SATA interface. Should run you about $30.
Antec 300 Case is great:
Phenom II triple core... best deal on the market right now
Hard drive is pretty self explanatory. Try to get one with at 32mb cache if you can... but a 16 would be okay. Aim for 75 bucks and you should be able to get 750 Gb capacity.
Memory: Geil Black Dragon... lightning fast timings. Wow, what a deal
Motherboard: This is only a suggestion, ATX makes it easy to plugin and the 790 is a decent chipset, although a 790 FX would be nice
Powersupply: a 650 watt or more would be more than enough. Modularity would help you out in the build. 75 bucks, give or take. Do NOT skimp on this... I speak from experience. Mushkin makes a great PSU. Quality! Read the ratings!
Video Card: Radeon 4870 1gb, overclocked
That should be about it. Don't get a sound card. This thing will actually rip apart anything you throw at it, don't be fooled by the price. Need help assembling feel free to email me - and good luck man! I took the plunge when I was your age, but dropped like 2.5 k so.... lol less risk ftw.
Keep in mind Vista's another 100 bucks, so use your XP license till Windows 7 comes out around christmas.