Can I use my Mac Monitor as a monitor for my Xbox?

It depends on which one

Does it have HDMI?

Honestly it's faster just to plug it in and see

IF your monitor has no HDMI or component then I wouldnt bother. Your monitor has DVI, and you can get an HDMI to DVI cable, the problem is you wont have audio to the monitor, you'll need another audio source

www.monoprice.com/.../product.asp

I just wanted to throw in that you could use your pc audio as well if you'd rather not have your monitor and TV on at the same time. You would just need the following:

1) Xbox Audio Adapter

www.bestbuy.com/.../4561394.p

2) 3.5mm female/ RCA male Y-Cable

www.cablestogo.com/product.asp

You might be able to find different prices on both, or even at a local store. I just needed examples of what you're looking for to establish audio.

you can't use your computer monitor. You can use a MONITOR - TV with a scart or hdmi source

I'm not trying to brag but I use a PC monitor when I'm on the Xbox, I use a XBOX to VGA cable, they are real cheap, but for audio I use the Astro's headset so depending on what audio source your using depends on if you need the cables above.