In reply to williemiller:
Twenty miles each way is a *lot* so I'd build up to it and even give yourself a day off each week if you have an alternative. My commute is 13 miles each way and I can manage five days a week most weeks through summer though today I felt knackered so got the train.
OK, to bikes. You'll definitely need mudguards and either panniers or saddlebag depending on how much kit you need to take with you each day. Try and look for a tourer style bike, they are set up to be more comfortable. Check with your LBS to see if they've any second hand ones that might be suitable, also it's coming up to the time of year when next year's models are appearing so many shops have big reductions in the current stock so that they can clear space for the new stuff. CX bikes are good in that they are pretty versatile - what used to be called "a bike". My wife has a Kinesis FiveT which takes panniers and which she uses for commuting as well, costs a Grand though so might be more than you want to spend.
Avoid going for the rucksack option as it will just increase the general sweatiness and level of discomfort. I'm assuming that you have access to changing rooms and showers at university. If you've secure storage try and leave as much kit as you can at college: shoes, jacket, the big heavy items. I only take socks, underpants and t-shirt each day. I leave a spare pair of underpants at work just in case I forget!
Is your commute going to be all on-road and if it is then is it all lit? If not then you need a more powerful system especially if you've got hills (descents) where you will be going faster. For me the ideal solution is a dynamo hub which powers front and rear lights but unless the bike you get already has one then you need to look at the cost of a wheel build to retro-fit it. They have the advantage that you can't forget them and there's no battery to go flat. Otherwise most light systems these days are pretty good, quite a few people reckon the Cree lights are fine but I've not used them myself. I'd look at having two sets of lights just in case one packs up, Moon Comet lights are small, bright, USB rechargable and have steady and blink modes - I use these as backup for my dynamo light.
HTH