I would go 1GB. I don't do much gaming but with the following phones Samsung G1 (512), G2 (1GB) and G3 (2GB). Less slugish with each beyond what I guess is CPU alone. My G3 can be set to clock almost as low as G2.
CPU, RAM and drive are the three critical components for performance. Depending on task anyone could be the bottleneck. Starting with better/more prevents/delays the onset.