When I played through Gothic 3 I did so on a laptop with very similar specs as yours (except I only had three gigs of ram, and was using Windows 7). It was usually more than playable but it never really ran "great" except in a few areas.
The most important thing is to make sure you have the latest community patch installed, the latest community patch is 1.75, though when I played through it was when 1.74 was the newest. However according to the World of Gothic website, the 1.75 community patch is supposed to improve performance even more.
A second tip would be to (using your mouse) look almost directly at the ground, it can seemingly instantly eliminate lag sometimes. This is usually most helpful when you're in a fight as the game can get frustratingly laggy then. I haven't seen anyone else mention this, so this could just be my luck, but give it a try anyway
This third tip may not sound very appealing, but run the game in it's lowest possible resolution. It really makes a difference, when I first tried it I was frustrated because I was in the process of trying to liberate a town, but the disconnect between me pressing a key and the nameless hero actually performing that action was around ten seconds, however when I made that change the game seemed to kick into fast forward. After learning that, I was able to turn the rest of the graphics options up to medium without losing any of the gains I had made performance wise.
My fourth and final tip is to just understand that developers maybe set their goals too high, and the game sometimes suffers because of it. It's a huge map, around the size (though it feels much larger to me) of The Elder Scrolls: Oblivion, only it's much more populated (you will find entire herds of animals sometimes)and everything is loaded once. The desert area Varant generally ran fine for me, only lagging in areas after a city liberation. Nordmar (the snowy tundra area) wasn't exactly completely laggy, but it was far from smooth. Mrytana the plains/forest region of the map ran the best overall, outside of the direct area in which you start the game.
I hope some of these tips help, it really is a great game underneath the bugs and stuttering.