I have a bad valve in my stock 415hp Volvo Penta 502 and figure I might as well replace the cam and heads with high performance parts.

What is the biggest bang for the buck?

That is the same base engine as the Merc 502 Magnum, but you have the advantage of already having an Arizona Speed & Marine intake, so it's a good base. The heads must be matched to the cam, intake, and exhaust, so you have to decide what your goal is before selecting heads. You can probably make some nice gains simply by having your existing heads worked on by a good porting shop.