What's your power supply? And, what about the cooling?
As for the monitor, that of course depends on the games you play and whether you want to keep on playing new titles in the years to come. If you want to play Crysis, for example, you can use all the power you can get, regardless of the screen size. I have a 17" CRT myself, and I wouldn't mind having an 8800GT. The cardinal rule of PC gaming is that given enough time nothing is ever overkill.