13" w/ a SSD upgrade.
Unless your doing processor intensive stuff like rendering then the SSD makes all the difference in the world.
I found my 13" Macbook 2.26 Ghz with a 64gig SSD just as useable as my Quad Core 6600 w/ RAID 0 on a pair of WD Velociprators.
I just switched to a MB Pro 13 and could not believe how long I waited for it to boot compared to the SSD.
It runs Office and CS5 great, so long as your not doing rendering, and I'm sure it'll render fine but if you are doing things specifically around the processor speed then obviously you'll need more. My desktop would crush the MB Pro doing video rendering but since it was no longer a requirement I switched.
I went to a 13 MB Pro as a desktop replacement, I'll pair it with a 30" monitor and it'll do what I need.
I do travel a lot with my laptop so the form factor helps too.
My biggest complaint about the 13 is the 1280x800 but was before I started to use it. On a Windows machine 1440x900 is the bare minimum. I hated my wife's 1280x800 laptop and thought the same of the Mac before I started to use it. 1280x800 is perfectly useable although I'd love to be able to get a 1440x900 even in the 13".