-Thermaltake are great Powersupplies. DON'T skimp on your power supply!
-You don't need Quad-core and that's not "budget."
-If you're not into overclocking and really messing with your system very often you don't need a high end motherboard or high end RAM. GSkill RAM is cheap (last time I checked) and I've been using it for a year now w/out a hitch. Save money here. A 775 mobo by a good manufacturer is going to last you a while, and as others have mentioned SLI or Crossfire isn't really necessary (and doesn't appear to be in your budget).
-HDD are cheap, getting a new doesn't cost much and they do affect performance. You also may have issues putting your old HDD onto a new motherboard, IIRC XP won't run. (Am I wrong on this one? Doesn't it think its being installed on a new PC with a new mobo?) If anything port your DVD burner over rather than your HDD.
-Often, paying top dollar for the newest GFX card is a waste of money. Read the reviews and get the next best, lower tier card. (Odds are it was almost as expensive 7-8 months ago.) Then you're actually getting a good performance per dollar ratio IMHO. Tom's Hardware is a great place to check these reviews out. (For reference, on my C2D 6600 w/ a 7600GT ($80 1 year ago) I'm still running CoD4 at 1024x768, albeit with most of the eye candy turned off. It still looks AWESOME however, and the 7600GT is obsolete these days. So, don't be too concerned with the best thing around. If you want to save money that may not be possible.
Have fun! I did the same thing a year ago and it was a blast and I learned alot.