Re: Upgrade from an 8800gt?
Monitor all the way. I wouldn't go for anything smaller than a 24" either. This is one upgrade that should last you through more than one pc build and the scaling issues are, for the most part, a thing of the past. I have a Dell 2707WFP with a native res of 1920x1200 and run most all my games/sims at 1680x1050 with an 8800GTS (G92). I get great frames and normally run 8xAA, 16xAF globally.
I don't play (or even have) Crysis, but UT3, COD4 and Oblivion all run flawlessly along with my flight sims (IL2:1946 and LOMAC) and while I'm not running the highest res, the AA settings and the large screen real estate make everything I do on it a completely new and wonderful experience.
I upgraded from a 20.1" 4:3 LCD and the difference was nothing less than amazing. You can get a really good 24" monitor these days for less than a GTX280.