RAM will affect render speed only if your current renders are getting swapped to disc - in which case no GPU is likely to help.

I have just bought myself the same card and while I haven't had much chance to push it it seems substantially faster (probably 2 to 4 times faster) than pure CPU, an i7 920, so as long as your scenes fit into 4GB it may well be worth adding. Of course if your scenes need more than 4GB of RAM (minus any used by the system) then the GPU won't help anyway.