Linux will be fun for you. The GCC compiler suite handles the C++ language very well (unlike certain versions of MSVC which don't tend to behave quite as the language says they should) - and I'd recommend you look into C++ rather than C, because object-oriented languages are becoming more and more common nowadays; so class theory - along with inheritance, polymorphism, and all the trappings - will be more useful in the long run than pure C. It's also a piece of cake to write C code if you know C++, you just have to try and remember which things you're not allowed to use.
Oh, and once you're happy with the language, look into SDL. Uplink uses it for basic graphics services under Linux; it's a free library which handles 2D graphics pretty well, and also provides a stepping-stone to OpenGL if you want to get into 3D. It's also cross-platform; a game I wrote on PC using SDL only took about four hours to port to MacOSX.
Katana Steel: Well, the DevCD is £30... it *is* quite intimidating when you've got the whole project in front of you, though. I'd reckon that a total rewrite would have to be a group effort, that it'd need all the DevCD owners to be working on parts of it (not least because they'd likely need to modify their own mods to cope with the new code).
I reckon we should leave the multiplayer for Uplink 2, personally. I don't think you can just 'stick it' onto Uplink 1, because it would unbalance the game (i.e. it would mess up the gameplay).
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