Like Death rattle said, nitrous would be the cheapest, about $500 + installation. Beyond that, a Vortec top end kit would probably be your best bet. You would have to buy a new intake though. As far as parts go, you would probably be looking at around $1300 for the upgraded heads, intake, bolts and gaskets, rocker arms, valve covers and cam kit. You could also get a set of iron eagles along with a new cam. You could use your current intake that way, but out of jegs, the heads are $1030, and a cam would still be around $200 which is what I assumed for the vortec combo. I think each of these combos would perform about the same, but I think the vortec kit would give you a snappier feeling motor on the low end with about the same max hp. The dart heads on the other hand, will give you a little more room to move, as people usually dont port Vortec heads. They can crack if they are heated up too often, which would not be that great if you just put $800 in porting into them.
Adam