mcdhu,
Welcome to Macintosh!
Any need for antivirus yet?
No. It invariably makes the computer slower and is *totally* unecessary.
The best way to transfer 20Gb of iTunes music from PC to Mac (it is backed up on a 30G iPod)
Migrate iTunes from PC to Mac
Office for Mac or Open Office or iWork for docs/WP etc.
For maximum compatibility, Office is the best choice, if you are exchanging a lot of work between PCs and your Mac. OpenOffice is free and does a really good job, but it still has a slightly "weird" feel to the user interface. iWork is the easiest to use with your Mac, giving very professional-looking results with the least effort. "Keynote" presentations are far, far easier than using Powerpoint (and look better imho), and adding pictures and other media to your documents simpler than in Office. You get a trial of iWork included with your iMac, so you can try it out and see if fulfils your needs.
Any need to tweak my Netgear DG834Gv3 to operate the iMac wirelessly?
No. It's the same as mine. Works great. Due to being on O2, who changed the way their servers allocate IPs, I had to put some non-Netgear firmware on the router so that it didn't drop the connection all the time. It was an O2 thing, not a Mac issue, though.
Likewise the Canon Pixma MP520 Printer which is wired to the main household PC(which will stay)?
The driver for your printer is included in the Mac OS 10.6 (which will be pre-installed on your iMac). It should work fine, both printing and scanning. You may have to check that the PC to which the printer is attached is set up to share the printer.
Canon Printer Drivers in Mac OS 10.6
Applemacster