pistons, heads, both?

IMO, if you're on a budget, and want to start going fast now, use what you've already got. It sounds like you've already got a solid bottom end. I'd take the magnum heads into a machinest and have them rebuilt, milled, ported, polished, and drilled to accept you're existing manifold. That way all you have to buy is lifters and pushrods.
You will get a SERIOUS increase in power, and spend less than you would on a set of new heads. The other big bonus is that you're car will only be down for a day because you won't need to tear into it until the heads get back from the shop and you have all the nescessary parts.
Don't forget all the extra costs aside from just the cool parts. You're gonna need a head gasket set, oil, and antifreeze at a minimum. When I swap heads, I usually to a full tune up and oil change at the same time (It seams cheesy to me to put old plugs in new heads, or new oil into an old filter). It can be done cheaper for sure, but ,my experience has been that changing heads for me usually costs a couple hundred bucks in tune up, oil,antifreeze, filters, gaskets, and misc. supplies.