~$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)