Red Armstrong uses ported iron heads on a 4.1 Stage block and he seems to run some good numbers. Very, very steathly. You can obviously do better with M&As or GN1s (or someday T/A's).
But if you want to upgrade as money allows, using iron heads might be a good way to go. Build the bottom end "right" with all the good parts, and change the bolt on items when you can afford it.
This strategy will work if you use Stock style (Non-Stage 2) heads. If your plans include S2 heads, you need to buy pistons(and lots of other expensive stuff) specific to the S2 heads to get the compression ratio correct.
DR