Help me build a PC...or two

Well, how about I throw down some things I'd like to have and see if you all can fill in the pieces or perhaps a cheaper yet similar equivelant?

It's a bit overkill, but I'd like an Intel i7 920 (http://www.newegg.com/Product/Produc...82E16819115202) for a processor. Looks like $290.

Radeon HD 5850 (Cypress Pro) 1GB Black Edition 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card (http://www.newegg.com/Product/Produc...82E16814150454) $340

G.SKILL 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Triple Channel Kit Desktop Memory Model F3-12800CL9T-6GBNQ (http://www.newegg.com/Product/Produc...82E16820231225 $165 for good memory, not sure if that would be overkill. Or if 6 gigabytes is too much.

I'm looking at $800 already. What motherboard would I need to be looking at to go along with that setup, and where can I afford to downgrade some while still maintaining performance that will last 2-3 years?