both the things work together.... RAM + Processor + Graphics card

all are one part... when compositing the size of the scene is what matters.. your ram will be eaten by after effects.. and the rest is processing power.. .AMD's are good at such places...

for example if you start rendering your animation and stuff, then one thats like 3 mins, 10 mins etc, ,,, will kill your CPU...

and in maya.... ram is very important..... and equally is processor... you will need atleast a 512 megs to work properly with maya... i am not saying that you can work with less than that... but 512 megs is good minimum amount...

for compositing a 256 would be fine... but you need processing power there... unless you are a buddhist monk and have a lot of patience, you will need to get a very fast processor...

hope that helps,
Tariq