Just received my Vostro 1500 (it took only 4 business days!) and I'm loving it. People say that Dell's quality has slipped, but this notebook feels solid. I certainly recommend the 8600M because the 8400M is really just a low-end card. I mean, if you're just looking for a quick game or two a week, then the 8400M would probably serve you well. If you really like your frames, go for the 8600.
The thing about the Vostro is that you can configure one with XP Home/Pro and NOT Vista. That was the major selling point to me, and I'm quite happy with the decision. The only real gripe with this laptop is its heft. With the lid closed, I would estimate the weight to be at least
8 lbs. Otherwise, I love it!