There are lots of SFF cases that fit 8800 gts, GTXs on the other hand tend to need modding to fit. SG01E, Thermaltake lanbox, and alot of other Matx's will fit a 8800gts no problem. Some shuttles require the side of the case to be modded a bit. I have a SG01E that has a 8800gts and it friggin rocks.
If you get a good MATX board you can indeed overclock reasonably with no adverse effects if you use a good hsf and maximize airflow. I have a 4300 running at 2.7 at the moment and it could go higher than that If I need to. Dont over spend on a proc thinking you cant OC a MATX. Sure its a bit harder thermally but it can be done and save you money to invest in better components elsewhere.