Well if you have a motherboard that supports Phenom II CPUs, you can just get one of those and be OK.
If you want to upgrade your mobo + CPU, then you definitely want Intel. Get a decent z77 board for $100-150 and pair it with a core i5-3550k. That's a ~$350 upgrade, though.
it's okay i have the money so can you show me a good mother board?