are you gonna get a full swap? or just the motor?

if you are limited on cash, hit up the z6.
i've seen good power come out from these thru the basic bolt-ons(i/h/e) and some other minor upgrades(chipped ECU, cam, etc).
if you want a lot of power, DOHC is the way to go.

really, you need to look around and see whats out there and what kind of deals you can find. like the guy above me said, you're asking for opinions, and what can you afford.