It depends on what type of web development you are going into.

If you are looking to do html/css development only you will need a windows machine or a mac that can run windows in parallel.

Also, I think you are better off getting XP rather then Vista because getting multiple versions of IE running will be easier. Google Multiple IE and you will see how easy. This is important for cross browser testing.

If you are doing programming in php or perl or ruby it may be nice to run Linux as a dual boot and run a development server on your development machine. This is what I do. I boot to windows to do my html/css and photoshop then hit linux for the real work. But note, OSX has a large support in the web development field with its easy to set up development server as well.

If you are going to school for web development (only one place I know has this as a major) I would recommend talking to the department chair or some older students in the program.

My personal recommendation is to learn Linux and embrace open source. Failing that, get a mac.

In terms of hardware, I am looking to buy a laptop as well. Important to me are three things.

1) screen size. In web development the more screen you have to look at code, browsers, etc the better. I develop in multi-monitor environments.

2) Processor. I will be getting a core 2 duo running at 2Ghz, minimum. But I am once again running a server as I develop and tend to have more applications open then a html/css developer (altho they still have a lot open!)

3) Ram. Ram is cheap now a days. Get 2GB at least. With these processors I think ram is the thing slowing you done more then anything else.

I will keep tabs on this thread, if you have any questions.