remember that you only have 120GB on that SSD, so everything else that are not on the SSD is gonna be WAY more slow.
but having the rest of the data on a RAID0 setup reduces the difference in speed.
still the SSD will kick the RAID0 setup's ass.
you don't "need" it, but it is just great to have.
if money is a problem then remember you safe a little on the RAM by going 2x4.
it's just what I would have done.