What do you mean by portraits? Flattering headshots of your gf outdoors? Character studies of your grandparents? Indoors? Outdoors?
A TTL/ETTL unit like the Canon 430/420 or 3rd party equivalents would be a great general purpose flash, but hardly the last word in lighting control. If all you want is a little daylight fill flash, they will do fine (your pop-up would be okay too). They're great for general indoor lighting too.
But if you're really interested in learning about and playing around with portrait lighting on the cheap, you might want to consider a couple of inexpensive manual/auto flashes (no TTL), light stands and modifiers (umbrellas are easy). Perhaps a small slave flash with a home-made snoot for a hair light.
I have a couple of old Nissin hammerheads and a Sunpak 383 (love this flash!) bought thru B&S for just this sort of thing. The Vivitar 283 HV is also a popular choice, although it normally sells for more than the Sunpak (check the trigger voltages for older flashes though -- some will fry your DSLR. Google if not sure)
It's more work, but ask anyone who's tried -- getting the flash off the camera and using light modifiers such as softboxes / umbrellas gives you so much more scope and control. A single flash on your hotshoe, no matter what fancy modifier or wall/ceiling bounce doesn't compare.