As a college student with a 17" Mac at a school that gives us free IBMs, here's my advice:
1. Buy a Mac. Sure, I'm biased. However, I've had one issue with my Mac. Hard drive broke after one of my friends dropped my backpack six feet off the ground. Surprise? Everyone of the kids on my hall was into IT at least once with their IBMs. Most got viruses. Almost everyone struggled with setting up. Battery life is an absolute joke, I get five hours with mine - some of their batteries have already degraded to 45 minutes. Imagine what it's going to be like in three years. I could go on if you really want me to.
2. Wait to buy it. Apple runs a promotion over the summer for students. You get a free iPod touch and printer with purchase of a new Mac. Best Buy will often match this deal and you get student pricing which is $50-$150 off. While a new MacBook will set you back over your budget, you could sell the iPod touch and get very close to the price.
3. Bags. I like backpacks. Check out STM and BBP Bags for both backpacks and messenger bags, they are all well made and aren't of the booq pricing.
4. I love my 17", but 17" PCs aren't the right choice. The weigh a ton and get ridiculously hot. Combine that with crappy battery life and you are in for a whole lot of pain. If you go PC, I recommend the 15" range (or 14.6") - seems to be where the best quality exists.
I'll have a Mac Back to School guide posted towards the end of this month with a lot of info.