What impact can a dual monitor setup have in overall performance?
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.