I'm also a bio major and have plans to attend med school! I knew right away that I wanted a Mac.
Today everything is so compatible with Mac or PC but nothing can give you more options than Apple.
I bought the Black MacBook in August of 2006. Never had any problems with it but as my pictures, videos, documents, and files grew exponentially I ran out of room. This January I upgraded to the brand new MacBook Pro which is an absolute Godsend. I can do everything on it and I still have so much space and speed. The thing I love most about my Mac is the user friendly compatibility. Everything is customizable to how you maneuver. Also, if you have a problem with your computer you take it to the store. No need to sit on the phone trying to fix it yourself and then possibly have to ship it to get it fixed. Apple has always been great with advice, tech support, and even general dumb questions.
I will admit I am very Mac biased butttttt....if you want a PC don't get a Dell. A Dell can be a good computer but they are also built to break. If you want the discount and a Dell is your only option for a PC then you have no choice. But if you want a PC buy an Asus. They're an amazing company from the UK that just expanded here. They make a great computer and offer a lot of great features.
Another thing about Macs. The new MacBook comes in a solid unibody aluminum housing case. This makes the computer EXTREMELY durable and scratch resistant. However should you get a dent in your case (stapler falls on your comp) It costs $1000 dollars to replace the case. I just had a friend have that happen to him on the old MacBook and luckily it was only $200. Macs are really expandable as well. You have so many features that you can plug and play with. I would recommend Mobile Me, even if you don't have an iPhone or iTouch. It runs at $69 and is a wonderful option for email, online storage, and sync-ability. You receive 10-20GB of online storage which I use to put documents from my last semester on so my computer isn't slow and also things I want to back up incase my computer fails. You can access the online storage from anywhere and its MAC/PC compatible.
Any other questions just ask. I know a lot about Mac.
Sparknotes: Get a Mac.