You should evaluate this with the Activity Monitor. If you see a lot of page faults, then the additional memory would help. But you also may want to consider replacing the hard drive with an SSD.
You really should have at least 4 gig. Just installing that
much will improve performance as 2 gig is a very minimal
requirement, especially for Lion.
My rule of thumb has always been, for any computer,
"install as much RAM as you can afford".