Yes. 2GB is plentiful for any normal user. You're saying increasing from 2GB to 4GB will yield a definitive performance increase, especially with gaming. I have never seen definitive proof that the OP's games or general computer usage will see any noticeable performance gains going from 2GB to 4GB. Just adding RAM to a system does not automatically mean the performance will increase.
Ummm, actually, like most Unix OS's, adding RAM will pretty much improve general performance. Mainly because more RAM is available to the file system cache. Also, it's much less frequent that the OS has to reclaim memory from the cache or "dirty" memory that's no longer in use. All of those operations take CPU cycles, context switches, etc.
It also improves performance in allowing more applications to run without paging memory out to swap which resides on the physical hard drive.
Though I take all benchmarks cun grano salis:
http://eshop.macsales.com/shop/apple/memory/Macbook_Memory_Benchmarks
Finally, as one who runs a lot of apps and VM's, the more RAM the better. More people are doing that these days, and considering the rather low price of RAM, going to 4gb just makes a hell of a lot of sense. (Actually, I'm up to 6gb in my MBP).
Finally, as a Unix engineer who spends a lot of time dealing with performance issues, it's true that for any given specific problem, throwing more hardware at it may not fix the problem. But it's also generically true that more RAM is much more likely to help performance. Mind you, CPU, GPU and Hard Drive all come into play.