I use my laptop for near enough everything at Uni. I use a small 13" laptop which I find fantastic for carrying around and taking into lectures. At Edinburgh Uni, all our lecture slides are put onto the university system before the lecture actually starts. This means you can access them whilst in the lecture theatre through the university Wi-Fi. I have found this very useful as it means I can annotate as the lecturer speaks and I don't feel the need to write everything down from the screen.
It's becoming increasingly popular to use a laptop in lectures; it still isn't the norm but as battery life becomes better and screens increase in quality more and more people are doing it. There is an ever-increasing number of netbooks in lectures. These are laptops with very small screens. I find these difficult to use as the keyboard is too small and the screen is sometimes hard to see. Also, with the proliferation of the tablet (iPad) these are becoming a more common sight. I'm not sure how easy I'd find one to use though and they still have a 'posey' stigma attached to them.
In Edinburgh, all the university buildings have Wi-Fi, which means you can access the university network everywhere and in some locations the printers as well. This is great when it's exam time and the library is packed. People wait for ages to get a computer but because mine is mobile and has a good battery life I can take it in and find a seat wherever I like.
There are a couple of downsides to having a smaller screen though – with the amount of texts that are being made available online to read rather than handed out on paper, the smaller screen does make it more strenuous to read. It also makes life harder when you're trying to watch iPlayer or a DVD with friends back at your flat as gathering round a small screen to watch a cool movie isn't much fun!
I have a large hard drive in my computer (500GB) which means I don't need an external one but having plenty of space is becoming increasingly important. You need somewhere to store all those legally downloaded films and MP3s. You've also got to remember that important academic programs such as Football Manager take up a lot of space...!
More seriously, it's a good idea to get the Microsoft Office package; the student price is only £75 and that provides nearly everything you'll need. It's also really convenient as most universities will use these programs too so you won't have any compatibility issues.
If you do decide to go for a laptop, and let's face it, you probably will, I would recommend getting a couple of extras to go with it. A USB keyboard is usually less than a tenner and makes it a lot more pleasant when writing long essays. Printers are useful too; you can get an all-in-one printer, scanner and copier for less than £40 nowadays and are perfect for printing out drafts of your work.
I'd recommend a laptop similar to the Toshiba L730; it has a great HD screen, a large 500GB hard drive and at less than £450, I'm not sure you can go far wrong. It also has 2GB of memory and a great graphics card in case you do want to use it for games during your revision breaks!