It will fit. uATX boards are just shorter than ATX boards. Witdh is same and placement of slots is pretty much same so you won't have any problems with motherboard
Agreed with Feldmarschall^ The HD 7850 is a great price/performance card! in the link for your rig it shows that you have a pci-e 2.0 so you can absolutely support a gpu. The only problem that you may run into is the fact that prebuilt computer cases are not really designed for airflow, however if you are looking into the hd 7850 they do run cool and are not very power hungry so you should be good.