Just realised that I am running 32 bit W7 on my quad-core desktop.

The W7 package came with 2 CD's, one is 32 bit and the other is 64 bit.

Can I re-install/upgrade to 64 bit and re-register with the same security key on the packaging ?

(I realise I will have to back up all my data)

yep, just back-up, and do a clean (full) install, the licence key is for both versions. Mind you, be prepared for 150+ updates

Cheers, it can run overnight.

I can't believe I was running 32 bit all this time without noticing !

To be honest, unless you do actually have more than 4 gigs of RAM, I'd save yourself the heartache. You only mention that you have a quad-core processor, which will run under 32bit quite happily anyway, you don't mention how much RAM you have which is the main deciding factor in whether to go 32/64.

also if you reinstall, and you have office, there is a trick to making that install the 64 bit version (it does the 32bit version by default)...

Then you have all of the fun trying to find 64-bit device drivers, I dropped back to 32-bit for the time being as it was easier to get the device drivers.

The last time I installed Win 7, it gave me well over 150, in fact it was more like 170 software (patches, security, device drivers & general software) updates... Just over 3 hours worth.

also if you reinstall, and you have office, there is a trick to making that install the 64 bit version (it does the 32bit version by default)...

Unless you've any specific need to install the 64 bit version of office, I'd stick with the 32 bit version.

Just built a new machine running Win7 64. Took about 3 days before it stopped finding hordes or patches.

Still having issues with printer even with correct drivers. Apart from the printer and patching it's not been that bad.

As mentioned unless you've got more than 4Gb of memory you'd be as well sticking on 32bit. You won't see a perceptible performance benefit from 64bit.