Costum Build

I currently have an amd phenom ii x6 1045 t, radeon 6870, 16gb (1600mhz, corsair vengence) and i'm having some video editing issues.

I'm trying to see whats the loswest costing upgrades i could get in order to run either sony vegas or adobe premiere/after effects with little to no lag when previewing my work.

I can edit on sony vegas pro but it takes much longer then i need to be taking to edit. Right now i plan to focus on rendering in 720, but i want to have the security of being able to edit in 1080 as well if need be.

Does anyone have any suggestions?

I know getting expensive all around the board might be a better idea, but im trying to not go deep into my pockets just yet.i doubt i need to upgrade the ram, but maybe some suggestions on the cpu and gpu would be helpful.