Of course, more memory boosts computer's performance, but to put the exact % value is rather difficult as it depends on a lot of factors ..
The following should help shed some light on your dilemma:
"1GB or 2GB memory needed?" **broken link removed**
greater ram memory will definetely boost your speed ,but it depends on your usage sometimes it may not help at all as you already have enough,also if you have not yet installed new memory check for its speed and synchronise with the previous module else it will create problem...