Is my heatsink on right?

Hey guys,
I have recently installed a thermalright HR-05 IFX heatsink on my northbridge chipset in my GA-M59SLI-S4 mobo. I have a couple of concerns,
1. How tightly do you screw the heat sink down, i've got it tight enough so the heat sink does not wobble or move unless you touch it. I didn't want it to tight as not to crush anything :S plus the rubber washers on the back look like they could fall off if i screw it any tighter. It seems to be conducting fine when i touch the heat sink.

2. How hot should the northbrigde be running, i've used a temperature sensor aimed at the base of each heatsink(the onboard sensor has never worked always reads 25C). Most of the time its running at 45-50C and the southbrigde is ruffly at 45-52C, i'm assuming the northbridge should be the same temperature or hotter, is that a good indicator the heatsink is working properly considering i don't have a fan attached to it.

Thanks for the reply , good to know my temp is safe, but i don't think its possible to tighten it enough without damaging the chip. It takes moderate force to turn the heatsink but i don't think that can be helped as the metal clamp just holds the heatsink down where as it seems most stock heatsinks have the clamp physically built into the heatsink hence you couldn't turn it no matter how hard you try, I'd say i'm assuming the heatsink is mounted correctly if its conducting up to 50C of heat.

Thanks again for the reply though you've put my mind at ease.