Which revision of the Alienware system do you own?

Are you using the original motherboard? 

Nope. The best you can the X51 R1 (the computer I think you have based off what I have found with the motherboard name) is the i7-3770.

It actually would have been better to list the specs of your computer, though. 

Sorry. Yes I have the alienware x51 R1. i7-3770k, 16gb corsair vengence or whatever its called. lol. Nvidia GTX 750ti. I really would like to upgrade my CPU. What are my options? Would I be able to replace the motherboard. My wife got me the x51 a couple years ago when I told her I wanted a gaming computer. It's not a bad computer right now. I just would like a faster processor.

You can't get a faster processor. That's the best you can do unless you want to buy an R2 motherboard, the only other motherboard that will fit into the X51 chassis without problems..

You CPU is fast enough and will last a few years. Your GPU, though, is the one that needs upgrading.