The only patch you absolutely need is the official patch (v1.074) which I believe is already integrated into the GOG version of the game.
However, the UAP (Drog's unofficial Arcanum patch) is a tremendous bugfix, and even re-implements some items and features the devs had to drop for lack of time to get them working properly. The high-quality town maps patch is also quite good. Links to both are in the stickied thread at the top of this board.
The high-res patch, by contrast, has been known to introduce bugs (and sometimes outright break) some installs, depending on the system hardware. I play the game on a 23" monitor, without that one, and the game automatically resizes to fill my entire screen. Still, if you want to try the patch for yourself, there's a link for it in the stickied thread as well.