I have a tablet somewhere which runs a ULV 1ghz with 512mb ram.
It runs a hdd install of BT2 fine
Also, my eeepc (2gb ram) runs BT3b fine
So, I would say that, generally speaking, BT runs fine on lower spec machines...........
Of course, heavy number crunching will be a bit on the slow side.
But most tools will work fine.