Since your mobo doesn't allow OCing, a word of caution. If its a Dell or other "brand name" PC, make sure the CASE (and mobo) is not a BTX case. If so, you will have trouble finding a mobo that fits lol.
Easy way to tell is, when looking at the back of the case, if you mobo's back panel is above (or left of, if case is on its side) the expansion slots, you're good to go with an ATX (or mATX) replacement. If not, you will likely need a new case as well.
My PC is custom-built, not pre-assembled. I have a cheap Intel motherboard which really lacks in special features of any kind. I've got an HAF-932 so I could probably fit even an eATX.