Here's my opinion:
Play Gothic I, II +addon first. They're better and require less hardware.
Gothic III is certainly a good RPG. But it did not continue the style of its prequels, which are awesome. There's a heavy lack of atmosphere when compared with GI and GII.
Also, if you're one of those who play a RPG just for story, Gothic III is not your best choice. The story sucks imo. I've never been able to play through the game even once. And I'm a huge Gothic fan. But at one point, it just didn't motivate me anymore and I gave up.
If you're gonna buy it, one final bit of advice: Make sure you got 2GB RAM and a very good video card.