First of all, welcome to G1!
I started somewhat similar to where you are now, so this comes from my experiences.
I'm now, more than ever, a firm believer in the fact that it's the person behind the camera, and the glass on the front, that make the images. Lighting can be done with a window and very cheap home-made reflectors. With a single 430EX, while limited, if you get the flash off camera, and work to perfect your technique with that, you can learn the basics of lighting.
Next, add a 580EX (with your next block of cash) or 2, or a 580 and another 430, and you have a wireless 2 or 3 light system with full control on the camera. Now you can work on more versatile lighting, and improve still further. Then, once you have a solid grasp of using the gear you have, move into building a studio lighting set. You'll already have a small indoor system, and a highly flexible location set that carries easilly.
I went the other way, and started buying studio lights, but am finding that they are clumsy to move to locations, and require more power than I have at my disposal. So I am now adding the 580 and a second 430, and look forward to working with 3 portable strobes on location.
In short, go for the 70-200 F4L for sure over the 70-300. From the reviews I've seen it's a killer lens! Or, get the one I have, the Sigma 70-200 f2.8 EX DG. I use it on 20D and 30D bodies and have some excellent results. A little slower to focus, which is not an issue for portraiture.
These shots were with a single 430EX off camera with a set of cheap eBay wireless flash triggers shot into an umbrella. The whole wireless, stand and umbrella came from various eBay stores and cost me around $100 US shipped to Canada.
Good luck, and I hope this has been of some help!