Man, I'm in the EXACT same boat.
I've been debating between getting an iMac (once they update) or an external display for my 2008 MBP. My MBP is running CS5 just fine and that's the most intense program I run. So, since I'm happy with the performance of my current notebook, I'm leaning towards just purchasing a monitor. Then, if (and when) my MBP *****s the bed, I'll just get a new MBP. Or, if my MBP continues to work flawlessly for the next 2 years, maybe I'll just upgrade to an iMac in the next year or so and then have a great big two screen setup.
Now.
Which 26/27/30 inch IPS monitor? I definitely want to go bigger than 24". I know there are a lot of great 24" monitors out there, but if I'm going to get a big screen to edit on. I want just that, a BIG screen.
Any input is greatly appreciated and encouraged.
Thanks a lot.