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

https://support.apple.com/kb/HT3246 talks about memory usage for various Mac products, including the late-2008 MacBook Pro you are using. Your MacBook allocates a portion of its RAM for video. You can hit the wall with that if you're rendering lots of video, but as far as I can tell, your GPU is discrete (ie. not part of your CPU, so multiple monitors won't affect your CPU performance) and the amount of memory allocated to video doesn't change upon hooking up a second monitor (so you won't decrease the quantity of RAM available to your applications).

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.