Hey, I'm sure you'll find that Macs are brilliant

I've had mine since Christmas '09 and I'm never going back!
One thing that I would say is that, like with all software, make sure you keep your Mac updated. iWork will handle .doc or .docx files created in MS Word just fine, I think, but you can always use MS Office for Mac instead if you want. I personally prefer iWork but I use Office because it was bundled with my computer

The Mac OS recently had a large update to 10.6.4, as well as a new version of iTunes so I think as soon as you get your new iMac run Software Update to get them.
As to security/viruses and the like, personally, I'd say don't bother getting antivirus and a lot of other Mac users would probably agree. Provided you make sure you use the computer 'safely' - i.e. don't go on sites you don't trust, don't allow random stuff you're not sure about to be installed - don't open unsolicited Email - the like. If you get antivirus it'll only slow your Mac down, but if you're planning on sharing a lot of files etc. with Windows users you might want a free antivirus that'll scan for Windows viruses so you don't pass them on, as Macs can get Windows viruses but it doesn't affect them, and they can pass them on.
Sorry for the long post, but I hope it's helpful at least!
