I'd go for the 4870 at the price it's at, great bargain, will outperform the 9800gtx+. The E8400 is a great little CPU and should clock really well and if it's mainly a gaming comp, I'd definitely consider a fast dual core CPU. If you really want a quad core though then go for the Q6600 instead.
The 520W coolermaster will be enough to power the above rig, it seems to carry a good reputation and is also modular.
The P45 pro mobo is a good, reliable board. Should see a decent clock on the CPU. It also has crossfire capabilities, so if you feel you need a bit more power down the line, throw in another 4870.
The samsung F1 drive will be faster than the WD green, but will use slightly more power.
Also the 300 is a great case for the price, with excellent cooling abilities, which can be improved with the addition of extra fans.