I've built with the Zalman Z9 Plus case, it's got great features considering it is extremely cheap. It's not the best build quality, but hey, it works and it's cool.
RAM isn't bad either, like $30-50 for 8gb depending what you get.
PSU is a bit pricy, around $80-100 for something decent.
Win 7 is typically around $100, but maybe you can get a deal.
That leaves maybe $100-150 for a GPU, so like a 6850 or 550 Ti I guess. If you don't mind used, 5850s can be had in that range no problem.
Anyway best of luck.