Just make sure your subs work well in a small (probably sealed) enclosure.
The box has the PROPER amount of space(only use baffling to cheat if you must).
Get a good clean amp w/ X-over.
Give your amp ample power..good ground(important) and power wire with fuse.
Don't over drive your shiznit.
I have heard single 10's done properly sound SOOO much more powerful than multiple high dollar subs in poorly designed/matched boxes.
Don't be afraid of 8's. They can be awesome in the right setup. Hell, a lot home subs use 6.5's and 8's and fill a house with bass.
10's will work and one, believe it or not, may be enough if it done properly.
I have seen 12's.......most sounded like hell.
--- I use to get off on bangin' a single 10 and people asking what I had...."a 10" I would say. "10's, no way they sound like 12's"....."no, A 10!"...."shit, let me see".
Of course, this was after I went through an "all out:02redxbla " stage that it kind of sounds like you are getting into and that's cool. enjoy.
One more thing...DON'T NEGLECT THE MIDS AND HIGHS. I am sure you weren't going to.
And thanks GCountach for the info on WinISD. I was wonder when a "good" freeware speaker design program would come out(of course I haven't looked for a long time). I am going to check it out.