I'm not so sure if that's the best idea with a HD2900XT and a quad core processor. It'll probably run, but it's nice to have some head room.
For the RAM, get G.Skill HZs Newegg.com - G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
For the motherboard, the MSI P6N SLI is a good board, not sure if the Platinum version fits in your price range, though. Don't listen to Saltynay, you'd be better off with a 650i chipset, especially in a CHEAPEST BUILD. Why would you WASTE more money on something that gives no signifigant improvements over the 650i.
For a powersupply, here's a good deal, I think.
Newegg.com - mushkin 550200 ATX12V 550W Power Supply 115/230 V cUL,CE,CB,FCC - Retail
I say stick with the HD2900XT, unless you have the money to spend on the 8800GTX. But you're trying to make a "cheapest" build, so I say you should be fine with the HD2900XT.