Adding more RAM generally improves performance because the laptop caches less to the hard drive. I believe the recommeded minimum RAM for Win 7 is 2GB. I suppose on average the increase is about 6%.
Since the hard drives access is generally reduced due to less caching, battery life can possibly increase slightly with additional RAM.
I've added 1GB to my old IBM ThinkPad T40 which originally had 512MB of RAM and noticed a lot less caching, and a little more smoothness due to less caching. Since my T40 had two sticks of 256MB RAM, I had to remove one stick to install a stick of 1GB RAM. Total amount of RAM is 1.25GB.
If your 1GB of RAM is split up into two 512MB sticks, then you'll need to remove one to install a single 1GB RAM stick. Thus, your RAM will only increase to 1.5GB.