there are two scenarios here:
When you are doing the composition, because AE have to use ram to cache preview, so plenty of ram will help. But it's better if your GPU have Mercury Playback Engine support and massive VRAM, or else it struggles a lot for generating preview when you navigate across the timeline.
When you finish the job and submit to render, renderer cache the seq on the fly, not the whole seq at once. Ram became less a matter and raw processing power is the king. But x5650 is dated, power hogging and no faster than new i7, it is not so justified in long run.
If I were you I will rather invest on GPU for faster preview
guys, do remember it is DUAL X5650, multi thread performance should be better than overclocked i7. But X5650 single thread performance can be a huge problem.
And also X5650 motherboard has no SATA III.
If you taobxx, X5650 can be very cheap with motherboard, RAM, CPU and server power supply.