Will running a double monitor in clamshell mode put more stress on my computer? I'm wondering if my mid-2010 Macbook Pro will be more stressed than it should if I run a 22-inch monitor in clamshell mode.  It seems to be whirring more than usual when I do this, so am I better off just getting an iMac?
Not sure how to answer this objectively, but running it in "clamshell mode" is fine. I've been doing the same for a long time with my MacBook Pro with no issues.

If your external screen is bigger (resolution-wise) than your laptop's native screen, then it will of course need some more graphics power to run, but this is rarely an issue in practice.