If you're thinking about the V8 Swap, you should read this thread in its entirety if you haven't already -
http://986forum.com/forums/show-tell-gallery/54328-boxsterls376-introduction-ls3-conversion.html.
Personally I would source a used engine, maybe a 3.4 or 3.6, whatever 996 engine works easiest with your existing DME, wiring harness, etc.