Yep, what Angie says. If you can knit on two straight needles you can knit on a circular.
Whether or not you can stuff a bunch of stitches on the straights depends on the width of your afghan and how thick your yarn is, but I think it would be really frustrating to do it that way for a couple of reasons:
The knitting will get really heavy as you progress and will be hard on your wrists.
When you have a bunch of stitches on a needle, and you get to the end, it is really easy for them to pop off the needle which is very annoying.
I really encourage you to try with a circular. You can buy them at craft stores and they aren't very expensive.