I highly recommend the White Lightning/Paul C. Buff setups that Mark mentioned. There is not a better price to value available anywhere in strobe lighting.
We run a portrait studio, and I have five of the older Ultra 800 units which provide plenty of light for my work. Most of the time I use them dialed down to minus 3f. The larger units are quite powerful, but for portraits, you probably won't need it. I can dial the 800s all the way up and pull f16 from a silver bounce umbrella at about 8 feet, which is plenty for white knockout backgrounds.
These units were purchased used, and I have been wearing them out for years. They perform without fail. The only time a tube has blown has been when I broke it, and only once have I had to send in a unit for work. They covered the work under the original warranty even though I was not the original owner, and I had it back inside a week.
One of my clients just set up a studio in their warehouse for product photography, so we bought four new X800s for them. Plenty of power for all applications.
One head, a big white foam core reflector and a decent soft box is all you need to start. If you like a kicker light look you can add a silver reflector at 45 degrees behind the subject to bounce the main. (I do some pretty complex lighting but I still love pure simplicity in portraits.)
If you grab a Cyber Sync receiver and transmitter, you'll be all set: the transmitter has a miniphone plug that you can use to connect to the PC on your RB. Any other units can sync through the slaves.
Paul C. Buff: love them long time.