The best one is one you build yourself, because you know whats in it. Also roll-your-own PCs are almost always more upgradable that ones from a company like Dell or Gateway, which tend to be proprietary.
$500 might be tight. I dont have any current price listings from anyone nearby, so I'm just going to pull these numbers outa my ass...
Case - $50
Keyboard & mouse - $30
Motherboard & CPU - $200
RAM - $40
Floppy drive - $15
Hard drive - $100
CDR Burner - $100
Video card - $50
That's about $585, and you would need to add on some sort of sound card, depending on what you want. Obviously a faster CPU & bigger hard drive will pump up the price.