i ended up getting 3 420ex flashes (2 used) and an ST-E2 transmitter. i got two photek 60" umbrellas. my backdrop stand just delivered tonight. here's my results and some thoughts.
1. a 9'x20' muslin backdrop is HUGE. perhaps for 3/4 shots something 5x7 would be perfect. but oh well.
2. all the while i've been waiting to test this out (9 months or so, getting 1 piece at a time), i've been studying studio flashes. of course the big question was would a 420ex be able to give any light? then how powerful of a strobe do you need?
the GN on the 420ex is 23 meters at iso100 with a 24mm zoom setting, this is when the flash is off-camera (it auto zooms to a 24mm setting). the shot you see here had the apex of the umbrella 7ft from the stool/me. i was bouncing the flash into the umbrella (not shoot - through). the umbrella has a white reflective surface (not silver). for the shots i did, i was able to dial in +2 stop FEC, ended up using +1 stop for this shot. my iso was 100 and f-stop 4.5.
also, i used the ratio setting on the ST-E2 set at 8:1, which i think is a 2:1 ratio in normal studio strobes?
my conclusion is, it's very possible to get at least a 1 person portrait, probably full length , with a couple of 420ex flashes and still have a little headroom for FEC. oh, i used the 3rd 420ex on the floor by the stool to light up the background.
for me, shooting for fun, this is a pretty neat thing to do 
oh, one more conclusion. from all the reading i've done on how powerful of a strobe one needs, i think it really comes down to how far away you want your strobes from your subject and what f-stop you need to get a proper depth of field. i think this means that something like a couple of $150-$200 strobes would work well for 1-2 person portraits. i'll bet those alien bees for $225 would be HUGELY enough
i'm not going to change, at least not now. but hopefully this post will give people some ideas in their struggle for knowledge on lighting.
