For just net and lower end games you could build a s754 Sempron64 system for $400 ish...
And I'd suggest newegg, tho I am buying the exact setup I'd suggest from Mwave... I'll see how that works out.
Then as the mobo select the Biostar G6100 and at least 512mb ddr400 ram. that setup costs ~$173
Then buy case
Your on you own for a power supply, get the best one with leftover $$$ ask someone else, cause I'm in the dark psu wise.
Well, here's a good one someone recommended to me that's $40.
This is a good dvd burner burner
That's like $400 and then if you're going to play any modern games it could play them, other wise if you want get a 6600GT like this one,
It's prett **** good computer that will last for a while, get the gfx card I mentioned and it'll be good with games that come out this year. I'd also suggest 1gb ram instead of just 512, so get the extra ram or the videocard.