Aside from a rig, I have an Alienware M18x with dual 6990m's in it which I use as my gaming system, and for some supplemental hashing power.

When it's cool in the place, and the fans are all clean, I can get up to about 450kHashes off of the two GPUs combined.  CPU mining, as far as I'm concerned, wastes more power than it's worth. Unless you're mining at a pool which offers a PPS with a low difficulty of 32 or less (Scrypt), you probably won't get many shares accepted, and you'd probably want to hold onto whatever you mine for a while (of gaining value) to make it worth the power it cost you.  Imho, only GPU mining is viable, and only if you have a decent card.

You should also know that yes, heat will be an issue, as one day there'll be a bit too much dust, or it'll get a little too hot, and suddenly your mid 70's temperature is almost at the boiling point, and you very well could fry a card or worse.

Having said that, I accept the risks, and I keep my intensity down on my primary card (it seems to get hotter, quicker), so I'm getting ~230 out of one card and ~130 out of the primary card, and it's pretty stable.  I can even play some games without a problem while it's hashing at this rate.

Use a calculator to check profitability, a simple one is at dustcoin.com/mining