Comparing Call of Duty on my Xbox 360 and plasma television against my computer with a 20" Dell panel (TN, even), the computer wins, hands down.And I guess you need a really modern computer with a higher end graphics card to beat a console's graphic quality.
Regarding computer monitor: I know for a fact that many people use cheap 24" monitors (that cost around $300). I can't imagine graphics looking great on such office quality monitors.
But I may be wrong. I judge monitors at their ability to show photography and video well. The well defined pixels of a video game may be a different world altogether.
Regarding GPUs, you can get a very nice card for quite cheap (<$80) that will easily beat what the PS3 or Xbox 360 can output.
Also, if you're going to be wanting to do any kind of professional editing with photos, you'd want an IPS screen, and those don't come cheap. A plasma TV or a TN panel like my Dell don't compare.