A 22" or 24" widescreen is the bare minimum IMO for ANY monitor upgrade these days. I get a Dell (don't laugh, their monitors are wicked good) or Samsung.
Choice #1:
http://accessories.us.dell.com/sna/p...9&sku=320-6272
choice #2:
http://accessories.us.dell.com/sna/p...9&sku=A1319023
These are both 24" monitors (which is the defacto enthusiast point right now) and offer 1920x1200 native resolution. You don't have to run at that res, but it's nice to have it in the event you want to watch true 1080p HD content.
I have a Dell 2707WFP 27" monitor and run it typically at 1680x1050 for most games and at native 1920x1200 for everything else.
My .02c