In my opinion Jim there really is no need to look at upgrading the CPU yet, a Q6600 @ 3Ghz is not too far off the latest new CPUs to be honest.
If you find it struggling by all means look at the i5/i7 but my processor (Q6600) hardly breaks out into a sweat, coupled with the 4870 X2 (2GB) there really is no need to upgrade.
The only upgrade im looking at is an SSD eventually.
I suppose you could do with upgrading the 4870 or perhaps if you have a crossfire mobo just adding an near identical one
As for the 20inch monitor, its like anything, if you sat in front of my Dell 24inch monitor, you certainly wouldnt want to go back to a 20inch screen!
It all depends what you feel you need, I personally would shove in another 4870 (if u can pick one up cheap enough, and your PSU is up to it) as long as u dont mind the extra power consumption and noise/heat.