Protomotive Stage 1 turbo kit for a 3.2 will get you to 360HP with 6-7PSI of boost on stock compression/pistons, that's about the cheapest way to get in that ballpark and still have something that's not miserable to drive unless it's flat out. For example:
http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/585401-turbocharged-my-3-2l-carrera.html
Selling your car and buying a turbo already tuned would be about 25-30c on the dollar on the cost of doing it yourself. Just sayin'.
The 3.3 turbo and the 3.2 use the same crank/rods, and share the same achilles heel - the rod bolts. Once they're addressed, the bottom end is pretty stout, although past 600-700HP you might need to get fancy.
I'd suggest ARP rod bolts, FMOC, suspension and brakes should be addressed at a minimum. Really. Probably need bigger wheels to fit the brakes under, and you'll certainly want as much rubber as you can squeeze under the fenders. And every time you lift, you'll thank yourself if you fit an asymmetric LSD. BTDT.
G50 3.2 is actually a better platform to start from for 500+HP than a 930 (although 964 or 993 N/A would be an even better starting point), because there's more you can use so you end up selling less off - and a 5 speed is really useful; with good headers, turbo and 5 speeds, there's no turbo lag at all.
Big HP doesn't come cheap as you'd be pretty much reworking whatever you started with to get there, it's just a question of how much/how fast. I've heard that Turbo Craft can help you turn that money into shiny stuff pretty well.