4GB actually reduces your overclocks.
if you get multiple sticks, the memory controller is slowed down.
if you get higher capacity sticks, it means they pack more capacitors into them (it's tiny capacitors that store data in RAM), and more capacitors produce more heat, therefore don't overclock as much.
I would say at MOST 2GB ATM. 1GB will be fine for now, although 2GB is slowly starting to become more common (from virtually nobody I might add).
and yes, stick with 1 GTX.