Depends on your budget and what's important to you. An ideal monitor is one with good picture quality and good contrast. For gaming you will especially appreciate good black levels. Really just go to your local best buy, office max, and CompUSA to look at different monitors and see the difference in their displays yourself. Generally Samsung makes some good stuff but some of the upper end Acers are good as well. For gaming I say get at least a 20" 1080p monitor but certainly get a bigger one if you can.
As for the multiple GPU thing it depends on the type of games you want to play. Most games run just fine with just one good card. Of course if you want to play Crysis 2 with the DX11 patch at the highest settings then that is one of the cases where you want more than one card. Also, if you want to use features like 3 or more monitor eyefinity then having more than one card is often a good idea. On the nVidia side to use three monitor "surround view" you need at least two GPUs.
Umm a 20 inch monitor will do. For a gaming computer what is your budget? How much money are you willing to spend. I would suggest getting a $1000 to $1500 computer. You can custom build which I think can be bit cheaper.
ummmm, 24 inch monitor is what i always recommend, saying things like ummmmmm before you say something opposing another posters recommendation leads them to believe you are dis-crediting their suggestion just becuase you differ in opinion. uuummmmmmmm.