Just done this, and despite a few hairy moments everything seems to be running well, so far so good.
When I put the PC together I'd screwed the motherboard in too tightly and so some of the stand-offs came out when I was unscrewing to remover the mobo earlier. This lead to stress, which lead to the screwdriver touching the board a few times - but luckily not too forcefully
The fan was quite hard to pull off (edit: even after taking out the push-pins) and made crunching noised which wasn't too reassuring, and the brackets for the zalman heatsink were tiny and fiddly. I tried to push it on as straight as possible but in the end had to do it one clip (out of two) at a time.
Hope I applied the paste right, tried to make it a thin even smear.
It's WAY quiter now. I'm happy it's done and I don't appear to have ****ed anything up. Also tidied the wires up a bit, now they are just kind of messy rather than spread everywhere.
Thanks for the tips BTW. I feel a bit better about messing with PC innards now. What with this done and the faulty RAM replaced, the PC should now *hopefully* run nice and smoothly
*touch wood*