Ok well, asus board that have 4 ram slot ( dont need to go with a sli one or deluxe etc..), dont go with the 2 slot ( dont go mini atx motherboard) be sure it can support a little more than the 1333 mhz ram in case you wanna upgrade, the 1333 mhz is rather cheaps atm ( Kingston or corsair ) dont go with crappy ram you may regret it, you can go with a 120$ Pci express graphic card, if you go with amd, go with ati, if you go with penthium go with nvidia. 4 gig ram is enough. Win 7 64 bit as well, if you wanna save up, download a version and get a remove twat to make it legal.
I wouldnt go under a phenum x 4 go with whatever they have affordable with a good cache per core, 2 mb is good enough.
About the HDD , go with a sata and perso i recommend you western digital. They are generaly reliable, if you see a 64 bit hdd affordable go with this.
That pc shouldnt cost you more than 500$, give or take depending of the HDD and cpu you take. Remember sometime its worth to pay 20$ buck of more to get better, stronger pc will last you longer.
Hope it help you
Zuruss