For storage, you might as well go with a 2TB as those can be had for as low as $80 sometimes, unless of course you know you'll never use more than 1TB. For mid-tower, most of them are good today, so any that fit your price-range could be considered. Thermaltake and NZXT both offer some great budget offerings (Thermaltake A60 seems quite nice).
For CPU and the like, you might want to consider an AMD quad-core given the price-range, as they pack quite a punch but don't cost an arm and a leg. It's hard to recommend Intel's Sandy Bridge at the moment as the company is in the process of revamping some broken chipsets.
For GPU, the Radeon HD 6850 or HD 6870 would be a great choice, or NVIDIA's GeForce GTX 560. What kind of resolutions or games will you be pushing with your rig?
Edit: Are you looking to include a monitor as well, or is that already covered?