What impact can a dual monitor setup have in overall performance? I have a MacBook Pro late 2008 (2.4GHz Core 2 Duo with 4GB memory) and am considering getting a second 24" monitor. I understand having a dual monitor setup may decrease performance, but how much would that be? How can I know if a 24" second monitor would in fact have significant impact on my MBP performance without having to go to a store and test it myself?
I have the same setup - a 24" iiyama Prolite and a Early-2010 Macbook Pro with 2.4ghz dual core and 8gb of RAM. Originally I had 4, but I upped to 8.

I've noticed almost no change in performance since adding the screen - even running heavy apps like Photoshop, there is very little to no lag - and for general web usage (Chrome, Spotify, Forklift and Textmate) there is absolutely no change and no lag. 

I'm starting to think in hindsight that it was unnecessary to upgrade the RAM (although this was a £30 job from Crucial) - the only time where I notice the addition is in boot times.