You have a normal width case though a minitower and a PCIe 16x slot though it is probably an early version (V1.0 or so) so most graphics cards will fit but you will need to upgrade your power supply to run most of them.
Look here, select something decent and then check what its requirements are for power, then look for a decent power supply.
Do not spend too much upgrading a computer that old since it will never keep up with a modern computer. I personally believe that the processor is too slow for Windows 7 and it would have run faster on a clean installation of XP than it will on W7. You would be better off buying or building a new one for not that much more than you will be investing in that old one.
You have to be a little bit crazy to keep you from going insane.