My 2-cents worth:
Not sure how serious you want to get with this? I concur with most here that for now consider additional flash(s), stands and speedring/umbrell or softbox kits for use with flashes for outdoor work until you master it. Even if you moved on to lights/battery packs, you would always have times where you would want to use the ultra-portability of the flash setup. This can also be used with success indoors. The speedring/flashstand kits are inexpensive (and even dirtcheap on ebay, etc.)
As for indoors, monolights are your most economical/professional solution. If purchasing a "2-light" kit of any sort, consider getting minimum 300ws monolights if you can afford it (used equipment is an option as well.) The Alien Bees or Photoflex kits are good in this arena. Also, I highly recommend kits which include the travel bag that holds it all. Some of the shortcomings of cheaper kits are: lack of variable power, no cooling fan, plastic handles (which break/strip - especially when tightening to hold up a big cantilevered softbox!) More power is king - you can always dial it back! Be careful - this sort of thing leads to reflectors, grips, boom arms, hair lights, backgrounds for every occassion . . . . . .