I have a Rosewill PSU in my wife's computer and it does its job well. Its a 500W in their Stallion series of PSUs. Powers a i3 2100 and a GTS 450 with no problems at all.
That is the extent of my knowledge on their PSUs but every other Rosewill product I have gotten has been very nice for the money they cost. That includes card readers, fans, and cases. I trust their products.
When gaming, there's practically no performance difference between 4 and 6 cores so a thuban will be an utter waste. So we cut that ~$190ish part out and go for a $120 955be that will perform the same in games. The money saved will then go for a much higher quality psu and maybe even a 6870 if we save enough!
either way, never ever go cheap on power supplies. These are the types of things that can last 5+ years and go through multiple builds while other parts get "obsolete". Most programs dont use more than 2-3gb's except the very intensive "professional" editing of software making programs.
So 4gb is the sweet spot. Ram is cheap nowadays and it's price is only going to get lower.