I found this oldie but goodie Lifehacker article: When two computers are cheaper than one. It advocates buying a cheap laptop and building a desktop PC to meet your computing needs.
I think it makes a lot of sense. A few weeks ago, a coworker asked me what the most I would be willing to pay for a laptop. I hesitated, thought for a while, and said you might be able to convince me to spend $600. “Wow,” he said. “I’m considering a $3,500 laptop.”