what heatsink weight is considered safe?

i have a d975xbx in a foxconn case.
recently installed additional 3 case fans and now cpu is 39-41C idle and around 50 under stress with stock cooler.
was thinking about upgrading the cooler, but i saw that the original intel coller is ~380 grams and most if the coolers i looked at were over 700 grams.
would like to buy a thermaright ultra 120 (not the extreme) which is ~740 grams.
so i was wondering what is a safe limit theese days, considering my pc isnt really the one that stays in one place for years, i mean i regularly move it around its place, although carefully.

another question is that is the thermalright IFX-10 (the backpanel heatsink) worth it? i have a top case fan right next where it`s plates would end up next to the ram modules, so it would support it with fresh air from the outside

ok, thanks for advice, after looking around a bit i decided to stick with the ultra 120 extreme.

now my second dilemma is that i want to put 2 fans on the heatsink.
problem is that the motherboard has only 1 cpu fan header (4 pin), it has 2 other 1x3 chase fan pins, but also has another 1x4 fan pin (called AUX fan).
if if i set up both fans to be pushing air towards the rear of the case, which should i put on the cpu fan header and which should i use on a regular 1x4 power cable (the one used for hdd's)

You buy a fan controller. Or run one fan using the Mobo header to control speeds depending on temps. One fan should be enuff for your setup. If your really pushing it two is better.

If your pushing it you should have no problem buying a good fan controller so you can vary the fan speeds, depending on your usage.