My approach is that an Apple PC has an average work life of around 2-3 years (3+ years it'll probably have depreciated to the point of not worth bothering with anymore - my four year old white MacBook can't run OS X Mountain Lion, it's battery life is awful, and I really should get it recycled somewhere - an SSD might revive it for a little while longer, but I don't honestly think it's worth bothering with). I expect to get at least 3 years life out of this Sandy Bridge full-fat iMac that was purchased by work early last year.
The trusty old white MacBook is great for surfing porn, and making sure that your other computers are safe from viruses.
Keep your search history clean for prying girlfriend eyes, look impressive to her that you haven't searched for porn in YEARS, and then hide the white macbook behind the sink and go all animal every night.
(Not that I know from experience or anything, just overheard it from my brother's friend's cousin)