Most laptops last 3-4 years at most before the hardware shows signs of becoming dated. My dad has a 12 year old desktop with extensive upgrades, such as more RAM, additional usb ports, a dvd drive, and a new operating system (originally Windows 98). It's still painfully slow. That thing can't even play dvd's properly, it shows one frame every few seconds. I suppose that's to be expected with a then-fast 333 MHz Pentium II. Laptops are also more difficult to upgrade than desktops, because they use a lot of integrated parts to save space.
There are a few good and bad things if you want to get a mac. Though they are expensive, they also tend to be worth more secondhand than other used computers. However, you could buy a pc with better hardware for less than the mac. For example, compare the specs of this laptop http://www.newegg.com/Product/Product.aspx?Item=N82E16834220739 ($949.99) to some of the 15" macbook pro's ($1799), and you can see that higher-end macs are sometimes twice as expensive. The main difference between a mac and a pc is the operating system, not the hardware. Macs are essentially fancy pcs that run on OS X instead of Windows. If your classes don't require you to use OS X specific software, or you don't need a good graphics card for games/video editing/3d rendering, I'd say go with a cheap pc for now and hope for a better one in college. By then, you may have more money saved up from a part time job for instance. Some college programs also have software that only runs on one operating system. For example, CAD software is usually Windows-only. If you never run OS X on a macbook it is a waste of money.
List of Bestbuy laptops $330 - $600: http://www.bestbuy.com/site/olstemplatemapper.jsp?id=pcat17080&type=page&initialize=false&sp=%2Bcurrentprice+skuid&nrp=15&iht=n&list=y&sc=abComputerSP&qp=crootcategoryid%23%23-1%23%23-1~~q70726f63657373696e6774696d653a3e313930302d30312d3031~~cabcat0500000%23%230%23%2311a~~cabcat0502000%23%230%23%23o~~nf518||4c657373207468616e2024363030&usc=abcat0500000&ks=960&st=processingtime%3A%3E1900-01-01&prids=&cp=1&_requestid=314269
Whatever laptop you buy, I'd recommend getting a good warranty. With budget computers, there's nothing worse than facing a repair bill worth more than your purchase.