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.
Sempron 2800
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.
PSU
This is a good dvd burner
burner
Hard drive
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,
eVGA 6600GT
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.