I've been playing with these questions for years and have collected a few 45" umbrellas in silver and white, and couple 40" brollies (umbrella softboxes) . My strategy has been to bounce the flash(es) into the umbrella. But, like the Strobist, I've recently discovered the benefits of shooting through a white umbrella.
I would recommend two 60" Photoflex white convertible. Remove the black backing and shoot through it. This is a lot easier and cheaper than soft box with similar light qualities.http://www.bhphotovideo.com/c/product/42512-REG/Photoflex_UM_RUT60_Convertible_Umbrella_60.html
I did consider the 60" Photek Softliter, another "umbrella softbox", but at twice the price of Photoflex 60", the latter was a better buy for similar lighting quality. (Although, I'm still intrigued by this umbrella.)
The brollies that I do have are from PBL and are very good and cheap at $35 for two. But, the surface area of the 60" is tough to beat. The two 60" Photoflex are my primary light and fill, with the 40" brollies for secondary fill, hair, and backlighting.http://www.amazon.com/gp/product/B001BSQK8Q/ref=oh_details_o05_s00_i00
I often use 2-4 light sources. Usually 2-3 umbrellas and occasional 1-2 bare flash for backlighting. My third umbrella is most often one of the brollies, but I would also consider a 45" white Photoflex:http://www.bhphotovideo.com/c/product/42508-REG/Photoflex_UM_RUD45_45_White_Satin_Umbrella.html
For brackets, the SP Studio or Impact multibracket work great. I also have the Photoflex version of this bracket, but prefer the cheaper SP and Impact – because they're cheaper and I cant tell the difference in qualityhttp://www.bhphotovideo.com/c/product/50067-REG/SP_Studio_Systems_SPBRACKS_Multi_Bracket.html
Next, get at least two Interfit Tri Shoe Adapters. These work great:http://www.bhphotovideo.com/c/product/742155-REG/Interfit_STR141_Tri_Shoe_Adapter.html
For radio triggers, I like the CowboyStudio NPT-04. I have two sets for those time when a flash may be tucked out of optical slave range (and for backup).http://www.amazon.com/CowboyStudio-NPT-04-Channel-Wireless-Receiver/dp/B002W3IXZW/ref=wl_it_dp_o_nS_nC?ie=UTF8&colid=36ZD41KUFYMDR&coliid=IC7KTBT6BAK05
As for flash, skip the YN568. For the price of one YN568, you can get four YN-460-ii. I have six of these and may get two more. (I'm starting to shoot large groups and two 60" umbrella's each with three YN-460's would work great. With 8, I'd have a couple extra for backlighting.) With the YN568, you're paying for the ETTL. For studio work, this is a waste.http://www.amazon.com/Yongnuo-Flash-Speedlite-Yn-460ii-Pentax/dp/B003IZ9XTI/ref=wl_it_dp_o_pdT1_nS_nC?ie=UTF8&colid=36ZD41KUFYMDR&coliid=I2GSW9X8NFMMLB
I use 8' Photoflex stands which I bought many years ago when they were $45 . I like these stands and they work well with the heavier 60" umbrella. But, I'd consider something cheaper, maybe Wescott. Read the reviews on B&H or Amazon, look for comments referencing the bigger umbrellas.
Last note, batteries: Powerex NiMH 2700 charged with Powerex Maha chargers.