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
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?
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.
You might wish to consider the HD4770. It performs at 90% of the HD4850 when compared using an i7 920 CPU but likely your CPU would provide some bottlenecking so there wouldn't be much difference. The power consumption and noise are significantly lower on the HD4770 due to it's using the 40nm die.

Also, don't get a 1GB card. Your CPU and graphics card combo aren't powerful enough to run games at a resolution high enough to take advantage of more than 512MB so that extra Video RAM is just wasting energy.