If there are any specialized programs you need on either OS X or Windows, go with the Apple or Windows machine that it requires. If you're just going to be doing basic work like internet and word processing, you really don't need much power.
Keep in mind that an Apple laptop will cost more to purchase alone, a Windows machine will need internet security software (norton 2009 costs $69 a year, for example), and both will need some sort of word processing (Office 2007/2008 for Mac cost around $120).
Another thing, consider how portable you need your machine to be. If it is going to stay in a dorm room most of the time, for example, a 15" would probably be best, since they are the most mainstream and the best value. If you need portability to bring it to class etc. a 12 or 13" would suit you better.
On the Apple side, the cheapest MacBook 13" is $999, the older plastic model which has been updated to the specs of the newer MacBook "unibody" 13" ($1299). At the same screen size, you can get an ASUS laptop from Newegg for $1099 with a faster processor, more RAM, larger hard drive and better graphics card, as well as a 2-year warranty and a 1-year accidental damage (protects against Drops, Fire, Spill, Surge). The only possible downside? The ASUS may not look as cool as the MacBook (not made out of glass/aluminum), could be a bit thicker and battery may not last as long. But to me, that's not really worth it and a large 9-cell battery can be bought for under $100. That's only one choice, of course, there are MANY others.
At the 15" size, the cheapest MacBook Pro is $1999. Once again, for $1000 less, you can get an ASUS with comparable specs (some better, some less).
There's lots more choices like I said, but you have to decide whether you value pretty styling over value and performance, because generally, that's the way Apple and Windows laptops are (respectively).