The general rule for emulation is that greater CPU horsepower + more
Ram (up to some point you're not likely to exceed anyway) = better
performance.
More useable RAM always helps the computer as a whole, but it may not
do anything for Fusion or Parallels. I haven't done this for a while,
but a year or so ago I tried different allocations of physical memory
to Parallels, thinking that performance would improve fairly quickly
and then round off into diminishing returns. In fact, beyond a
surprisingly modest allocation, performance actually deteriorated! I
can recall the exact numbers, but is was somewhere around 512 to 640
MB that things started to slow down. At the time the optimal
allocation to Parallels for my setup was 384MB. On my MacBook Pro
with 3GB of memory, Fusion recommends that 512MB be allocated to
Fusion-WinXP.