+1 to "it's the engine, not the bike."

Regarding compatibility, figure that 8-speed goes with 8-speed, 9- with 9-, etc. Some hubs will work with anything up to 10-speed cassettes, but some won't -- check on that before you start buying anything else.

I'm thinking of changing a flat-bar bike to drop bars, and I was debating whether to treat it to some Dura-Ace & Ultegra stuff I've got. The thing is, it's 9-speed now, so not only would I have to use the shifters and both derailleurs, but I'd also have to get a cassette and chain (and maybe swap cranks, too) and a 10-speed compatible rear hub (or a whole wheel). I figure that I'll just get some Tiagra 9-sp shifters instead and be done with it; they're pretty good stuff these days anyway.

If you really want to have a good time riding together, I'd suggest to start thinking about a tandem.