You will definitely need to sell your LGA775 stuff to have enough cash. Go for i5, not i7. You won't see a difference in games, and look for the 'K' after the CPU model number (i3570K). The unlocked multiplier will allow better control for overclocking etc.
As for m/b, AsRock usually offer the best bang for buck. I would suggest a Z77 (LGA1155) board to keep CPU cost down.