No need of DDR3 memory, just costs more and LITTLE bit performance boost.. Not worth it imo.. (Unless you buy Core i7)
NEVER buy Phenom.. Phenom II is good but Phenom sucks... (Meaning if you buy AMD, buy Phenom II)
GTX 280... It's really good, but why pay from something when you can get the same with cheaper (HD 4890)...
600W+ PSU...? No way... One video card needs less than 500W... But I recommend Corsair HX520W... One of the best PSU, maybe even the best..
Like Wash said, you need a good motherboard... Think about only Asus, it's the best for making motherboards..