Intel Core 2 Duo E6850 for that budget.
Motherboard is tough to say depending on the setup you want. Plus new chipsets just out and newer ones coming soon, but if you must buy now on that budget.
EVGA nForce 780i SLI nForce 780i or
Asus P5N-T Deluxe nForce 780i
Although they might come in slightly over budget else.
Intel Core 2 Duo E6600
EVGA nForce 680i SLi