Being an advanced amateur myself, here's my experience with strobe lighting. I've got a pair of 350 Watt-second self-contained strobes from a Calumet kit (came w/ umbrellas, stands, case). I've added small & medium softboxes, a snoot, and 3 Pocketwizard Plusses (1 Tx, 1 Rx, 1 Tx/Rx). I do mostly studio work, very little location work so mobility/power hasn't been an issue yet.
Some thoughts:
1) Though 350 Ws is low for off-camera strobes, I've never needed to use the full output on the strobes when doing small studio, single model work.
2) Radio triggers are essential. Not only is a sync cable hanging from the camera obnoxious, it also makes it 100x more likely you're going to knock over a tripod with your expensive strobes on it or yank a camera out of your hand onto the floor.
3) Light modifiers are key. For me the umbrellas never really worked out. My favorites are the soft-boxes for soft light and my new snoot for very directed, contrasty lighting.
4) Off-camera vs Smaller (speedlight, etc.) -- If you plan on doing location stuff or journalistic stuff where you need to setup/tear down quickly then the more portable speedlight-sized flashes are what you need. If you want more traditional studio light and better options for light control (I imagine it's hard to put a 3' soft-box on a speedlight) then I think the dedicated strobes are the way to go. FYI: I believe Calumet sells a Travelite kit along with a battery for location shooting if you can pony up the cash, wanna carry the heavy gear, and have the time to setup in the field.
My stuff, lots of studio work:
http://bacmac.aae.uiuc.edu/ground0/groups_listing.php
