My best tip of advice is if you're on a budget, Don't buy a pc with a hard drive (you could save 50-100 bucks or so) use your current one (if its in good condition). Also sell your old PC in parts, ie the RAM, Graphics and see how much you can get out of it. Maybe 50 maybe 100 bucks depending how good the stuff is. Also purchase a 120gb SSD for you OS and put one or two games on it (maybe GTAIV LCPDFR ?!?!?!?!?) Thats what I did !
Also CONSIDER OVERCLOCKING ! Get a Corsair H80i or H100i (if your'e planning on buying a full ATX case because an ATX case most likely wont fit the H100i)
Good Luck !