LeRoy, I don't know where you live, but in today's Sunday paper flyers, Circuity City, Best Buy, Staples, Officemax, CompUSA, all have ads for oodles of computer components. At least two of those guys are sporting HD sales on their flyers.
I have had good experiences, in terms of quality, customer service, shipping, prices, and selection with NewEgg (
www.newegg.com) and often, though not always, they have some decent prices on all sorts of hardware and software.
I have had one experience, and it was good, with eWiz (www. ewiz.com). ZipZoomFly (formerly Googlegear) is also a good, reputable, and fairly decently-priced site (
www.zipzoomfly.com).
I'd really recommend that you either replace that 20 with a (much) larger drive or back it up with a biggie.
I have a 15 Gb drive, but it is on an oldie machine that my kid plays (literally) with... and HE has stuffed it full of stuff.