If you have one like this Micro ATX Intel Motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E168... , but if you have the regular ATX MB you can't.
Also most games Graphics card is big so check the card size first before you buy the new case.
Yes, if you use the SSD and fresh install the OS like Win7, it will make a big difference in like faster turn on and off PC, open the Photoshop program, copy and paste file, etc. but not for the games (expect load the games).