Dont you understand that the ram in the bionic and atrix phones is ddr-2 at 667 mhz, versus standard sdram which is at 133 mhz? I think 512 MB will suit you just fine.
Speed is not the issue, it's capacity. 512megs is still only 512megs. Apps will grow in size and complexity and so something has to be killed when you hit 512, and I don't know if compcache is the solution but it has a ways to go.