I have a 1520 with the 8600 GT. It's not bad, but it's certainly no head turner either for performance on games. I don't use it for gaming unless I'm in a pinch. If you're looking for a laptop that's going to be your sole PC and used for gaming, the 8600 GT could be disappointing. I installed XP to see if there was any performance gains over Vista. There wasn't any difference in performance that I could see. The screen and resolution is very clear and nice.
I really love my 1520, it's an awesome laptop. I use it a lot and the performance meets my needs. If you're looking for any heavy FPS gaming, I'd look elsewhere. Since I do my gaming on my XPS 410, it's not an issue to me. Since the 1330 only comes with the 128 MB graphics, I chose the 1520. The price was also better on the 1520.
I was disappointed that the XPS 1530 didn't come with a better selection of resolution. Due to that factor, I'd easily choose the Inspiron 1520 over it. If Dell ever changes that option, the XPS 1530 might be an alternative.
Good luck on your choice and hopefully I gave you some decent feedback.