I'm surprise no one mentioned Albion, a pretty good DOS RPG (the combat feels a bit like a tactical RPG, although the perspective is from First Person) with a Star Ocean esque story.
I tried it recently and it's very good.

I'm very surprised no one has mentioned Wasteland, Twilight 2000, Space 1889, Megatraveller ! & 2, Battletech 1 & 2, or Hard Nova. Granted they are pretty old, but still, some of those are classics.
ANd you call yourselves GOGs... Sheesh! :p

...or perhaps the Crescent Hawks Inception. That one kind of had the top-down sci-fi RPG thing going for it. Another old game that fits the bill: Sentinel Worlds: Future Magic. You might also consider Roadwar 2000 and Roadwar Europa.

Oh yeah, another vote for Albion and Arcanum - although none of them has Fallout 'feel', whitch is understandable, given the setting :D

Space Siege is mediocre. Being the Chris Taylor fan I am, I bought it, and I hate it.

Yes by Battletech I meant Inception and Revenge. :) The Mechwarrior games are more sim games than any sort of RPG.

I tried Albion about a year ago (never played it before) I like the 2-d top down perspective portions but when it went into the horrific semi 3-d sprite mode i wanted to tear my eyes out and couldn't keep playing it... story wise it seemed decent