is my gateway 500se desktop pc tower an atx case. How can i find out?

According to Gateway, your case is a uATX case, or micro-ATX. This refers to a standard for motherboards that came out in December of 1997. They list it as their 5 bay series on their website. You can go here, http://support.gateway.com/s/PC/2800071/280007101....

and see a similar case to yours. You can also search your computers components and such here;

http://search.support.gateway.com/iphrase/query?co...

Most, but not all of the micro atx stuff is unupgradeable, ie. emachines and small off-the-shelf computers, although you will find them now and then that allow an upgrade to agp video or an addon pci slot.

Gateway 500se

Form Factor uATX "like" form factor (7.4" x 9.35")

So the case is mATX ( micro ATX), smaller than ATX.