I kind of had little chuckle when I read this thread, because I am right now just finishing up putting a TPI engine in my '84, and now I get a sick laugh when I think there are others that will do the same thing
Bob is right that the tranny will probably give you a real headache, but the engine will not be a walk in the park either. I did mine based on questions I asked, and all I got were answers like "Go ahead, it's not that hard" or "That engine will slip right in and you'll have it running in no time". Well, the engine did slip right in, after all, a small block is a small block, but it's the wiring that Project_Speed mentioned that will be the most time consuming and frustrating part.
What it boils down to is how much do you love your current car, and how much work and money do you want to put into it? Like Bob mentioned, it will cost quite a bit to get that 6-speed into the C4, but it can be done. And yes, $5000 will be a nice down-payment on a C5, but will that make you happy? It's all a matter of personal opinion, just be prepared for a tough road ahead if you decide to do the conversion.