I like
Gary's Games, though my experience with them is local, not mail-order. They do a large business in mail-order bridge supplies, and they know their RPG's and wargames too.
As for GURPS, I think if you have GURPS 3rd Edition Basic Set, and the old printing of GURPS Space, there wouldn't be any need to buy newer printings of either of those. Some really good books though are the GURPS Compendium books, I and II, which include practically all of the useful actual game content from practically all of the world books, all in a nicely organized way. It was always a pain to have to consider buying a world book just to get a few interesting rules along with a lot of not-really-useful-for-one's-own-campaign-worlds background information.
There have been several interesting other books, too, such as the Traveller books, the tech books, and Vehicles (although the vehicle rules seem pretty unwieldy for a pen & paper game, even to me, Mr. Savage House Rules).
PvK