For LCD monitors it's best to leave RGB, contrast, and white point where it is, except for a few high end monitors.
I had the same problem with my 24" white iMac, so I calibrated it at 140 cd2, which is, by the way, recommended by Martin Evening in his Photoshop book.
Since then I had my LCD panel replaced, and now my iMac dims down to even under 120cd2 - got a better panel, obviously.
There is "Shades" to dim your monitor, but I'd go to the X-Rite site, as some versions of this software have a bug, which - isn't that great - ruins your calibration.
So, in your case, I'd rather go with 140 cd2 than with messing with RGB settings or installing dimming software that, so I gathered, has problems of its own.