I just finished upgrading mine a couple days ago.
ASRock H77 Pro4/MVP motherboard
Intel Core i5 3550 Ivy Bridge 3.3 GHz quad-core CPU
MSI geforce gtx 560 1gb GPU
8GB DDR3 RAM
Windows 7 64-bit
After rebates and taxes and everything, it was a little under $600. I already had a 400GB HDD and a solid PSU. Course, I had some difficulties that made me shell out some more cash for a new case and a new dvd drive, but that's beside the point lol.
I don't know much about hardware, but if you have 64-bit OS, you should get 8GB RAM. And if you don't, you should get one. I say this, because a lot of games are running minimum specs at 2GB RAM, and it's only going to get higher. Investing in doubling your RAM will keep you from having to upgrade your memory again in just a couple years.