If you have a PCI-E 16 slot than you can use any good graphics card you can find, however your limited by space in your case. Discreet graphics cards are often fairly decent sized.
A Mid sized tower will work for just about any PC build providing you dont go completely overboard. A full tower is a guaranteed fit for anything you could ever want and provides excellent Airflow
When connecting the cords from the PSU it is pretty simple. There is 2 Main connectors that connect to your motherboard the 24Pin and the Either 8 or 4Pin connectors. As for your Harddrives or CD/DVD drives there are (cant remember the name) little black connectors that are super super skinny that plug into both the Cd and HDD backs. And your Graphics card should use either a 6Pin or 8 ( or both) connector to power it.