Need some help with HD4580 and a PSU

im in a seek of budget gaming pc, and after alot of research ive dicided on those main parts:

Intel E8400
GA-EP43-DS3
G.Skill 4GB 800Mhz DDR2 CL5

because im on a budget ive decided to go with the hd4850 and not the 70
i can get the Gainward's HD4850 GoesLikeHell 512MB GDDR5 (is it the best 4850 i can get or should i stick with Sapphire?)

and i need a PSU, in my original specifications i choosed the Cooler Master 500W Extreme Power plus, but some say i should NEVER buy this PSU because its not worth it..

and they recomended the SeaSonic 400/500 (SS-400ET/SS-500ET)
is the 400W SeaSonic can power my system well?
im planning a little OC for the E8400 and maybe for the GPU so need to consider it aswell

and what do you think about my overall specifications for a budget gaming pc?

if you are going to OC, get the seasonic 500W PSU

The HIS 4850 got excellent reviews and it was in sale for 80$ with MIR.

If you can, just get one...

Also, not my matter, but I will give you my 0.02$ since what I am understanding is that you haven't bought anything right now.

1. You should maybe switch with an AM3 platform an a Phenom II cpu

2. Forget Gigabyte with LGA775. My board died 3 times in less than 16 months. It was a 965P-DS3. Great reviews, superb board but with zero reliability even without any overclocking. Ended up buying a Asus P5QL Pro.

3. Get a Crossfire ready motherboard and a 2nd 4850.... really awesome performance for the price.