I doubt you'll need a full tower case.
A mid tower case is usually more then enough for all but the most hardcore users.
The Antec 902v3 is an excellent choice.
Some might say overkill for your components but a good case is always a worthwhile investment.
You'll always have it for future build.
You could save some dough and buy a good $90 case and get a better gpu.
A 5770 or 460 would be a better investment then the GTS 450.
You have a cheap mislabelled psu that probably isn't much better then the unit that came with your prebuilt. http://www.hardwaresecrets.com/article/Cooler-Master-El...
It will undoubtedly run your system but i would set aside some funds for an upgrade in the future.