CPU: i5 2500k ($220 on Newegg)
MoBo: ASUS P8P67 PRO (about $178 with shipping on Newegg)
RAM: I honestly think 4GB is enough (especialy if it is at 1.6GHz)
OS: I would recommend getting Windows 7 if you are willing to change your OS (Vista is a RAM whore) for $95 on Newegg.
You'll have $50 left which you can spend on whatever you want