More is always good and as i suggest to others is if you have the funds now then do it now. In saying that though, with a 13" MBP 8GB would be a tad over the top.
I have the same model and with PS CS5, iPhoto, surfing, and with a few other aApps open i have never seen any lag. Even rendering in CS5 it has never been buggy or anything. I have thought to go up another 2GB RAM but really i have no need ATM so will save my $
My understanding is it will make the gaming experience better, but as Mac's arnt made for gamers then what is out there will justify what you have in your machine now.
But as i said if you can afford it get it now.
Dont buy through as with what they charge you could nearly buy a new machine or eevn a SSD ... Now that is worth spending money on lol
Goto
OWC ~ MacSales and follow the prompts for inputting your machine specs and they will do the rest for you. Highly recommended as i live in Aust and use this company and have never had a problem.
Cheers