~$450 budget gaming system upgrade

APPROXIMATE PURCHASE DATE: Over the next week or two BUDGET RANGE: Under $500 and as close to $400 as possible

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: gaming, surfing the web, schoolwork etc.

PARTS NOT REQUIRED: Monitor, keyboard, mouse, speakers, optical drive, OS, case (I think)

PREFERRED WEBSITE(S) FOR PARTS: newegg.com, 3btech.net (doesn't make a big difference to me)--
COUNTRY OF ORIGIN: U.S.

PARTS PREFERENCES: AMD CPU, Radeon GPU, DDR3

OVERCLOCKING: Maybe

SLI OR CROSSFIRE: No (although I wouldn't mind leaving the option open if I can do it with little sacrifice)

ADDITIONAL COMMENTS: It's about time I update my system (right now I have an athlon 2600 single core CPU, 1 gig ddr ram, radeon x1600 agp GPU.. 5 years old and not worth its weight in salt)
Thing is, I've got a really tight budget, so it's a bit of a challenge to come up with something decent. I'm just a casual gamer, and I don't really play anything too demanding but I would like to be able to max settings in what I do play (think Neverwinter Nights 2, Oblivion, etc..).

Here's what I've picked out so far, let me know what you think!:

CPU:
AMD Athlon II X4 620 - $100

GPU:
XFX radeon HD 4870 512mb - $132

RAM:
G.Skill 4 GB DDR3 - $70

Motherboard:
GIGABYTE GA-MA770T-UD3P AM3 AMD 770 ATX AMD - $75 ($65 w/ mail-in rebate)

PSU:
OCZ ModXStream Pro OCZ500MXSP 500W - $65 ($45 w/ mail-in rebate)

HDD:
Samsung F3 Spinpoint F3 500 GB - $55

Total: $497($467 w/ mail-in rebates)

I know changing this has no effect on the price but, this hard disk is faster than WD Black's but as cheap as the WD Green's.
Samsung F3 Spinpoint F3 500 GB
http://www.newegg.com/Product/Prod [...] -_-Product

PSU: Your PSU is now 65 $ w/o rebates w/ rebates 45 $

PS: ouch. Your budget is quite tight. I think the parts you have selected should be fine once you change the hard disk to the Samsung.

Thanks for the input, guys. I'm switching to the Samsung HD.

Dirtmountain- would the 400w PSU be sufficient for the HD 4870, or would I have to switch to the 9800 GT also?

And about RAM- How does this look?