What are good specs for a pc for WoW today?

Alrighty well by the end of Jan or begining of Feb, i should be getting a new pc, and i want it to be able to run WoW well. there lies my dilemma, im not tech savvy so i come to you, people of Y!A. i hear its cheaper to have the pc built as opposed to buying it already complete. the budget is limited up to..say $500. Yes i kno i wont be able to get the godlike specs for the comp of course, but its an estimate for affordability. i would jus like some suggestions for specs so WoW wont lag in 25 man raids on lowest settings. or maybe just minimal lag. Well ive given ya an estimate, ill let you people suggest away, also how much space on the HD would be good? cuz ive read you should leave at least 15-20% of ur HD space free...so if ya can throw that in as well, id appreciate it. Thanks in advance.