Hi Casey,
Firstly, A Very Happy New Year
The good news is that your budget of $666 is good enough for a new computer, but without a new monitor. You can also squeeze in Windows 7 Home Premium. Please bear in mind that Windows XP is on its way out and soon Microsoft will stop supporting it. I have stayed very close to the minimum requirements of SSE and kept in mind the processor clock speed of 3.0 Gz and the recommended graphics card.
To summarize your rig will be more of an entry level gaming PC. I will give all the details later if you approve this configuration.
Processor: AMD Phenom II Dual Core 3.2 Ghz $ 92
Motherboard: Asus $ 85
RAM: Crucial 2 x 2GB (4 GB) DDR3 1600 $ 76
Graphics card: Sapphire Radeon HD 4850, 1GB, GDDR3 $ 100
Hard disk: Western Digital 500 GB $ 50
Power supply: Coolermaster 500W $ 40
Casing: Coolermaster $ 60
Windows: Home Premium 7 $ 100
Total: $ 603You still have $63 left. If you can increase your budget to $ 700, I can recommend a more superior motherboard and a newer graphics card.
Regards
vin_sun