i7 920, Nice processor! I just love new pcs... gonna upgrade myself too soon, when the boss lets me... lol.
Choice of antivirus can be a very personal thing. I have always used freeware, and have not been disappointed. I was and am a big advocate of Avast, but of late I have been running Avira and it hasn't disappointed. It is lighter on resources and has a very high success rate with detection and removal... I have read a few reviews where it rated higher than most paid for AV programs.
I would recommend not to look at security suites as they can be very resource hungry (although the i7 probably wouldn't even sneeze at that). You can build your own. The firewall that comes with 7 is very good. Free Antispyware / malware applications are extremely good. Malwarebytes Antimalware and SuperAntiSpyware rate very high also.
All that said, the very first thing I do with a new rig is get an imaging program. There are a few free ones out there but I haven't tried them. I use Acronis True Image myself, it is the only security software I pay for.
I set my pc up exactly how I want it. All of my must have software is installed. Then I create an image of my system, a grandfather image. This is the one I want to restore my pc to if I ever get my pc in an usable or undesirable state with system bloat, unwanted software or even malware infection. There is also the option of creating father and child backups too... where the system is backed-up incrementally or differentially. The grandfather image (and ideally father / child) is always saved to an external HDD.
Anyways that's my 2 cents or 2 dollars worth.
I'm sure you will get plenty of tips from other users to take into account.
Have fun with your new rig. Sweet.