This was the tool I used when I got mine:
Buy refurbished. Don't build or buy new. The most cost-effective
Blue Iris computers are refurbished business-class systems such as "HP EliteDesk" and "Dell OptiPlex". You can usually get a good deal by searching [URL='http://www.ebay.com/']Ebay[/URL] for specific CPU model numbers, and...
ipcamtalk.com
I strongly encourage you to look at ebay/Amazon/MicroCenter or any certified refurbish site and by a used computer.
Many of these used computers are business class computers that have come off lease. The one I bought I kid you not I could not tell that it was a refurbished unit - not a speck of dust or dents or scratches on it. It appeared to me like everything was replaced and I would assume just the motherboard with the intel processor is what was from the original unit. I went with the lowest end processor on this list as it was the cheapest and it runs my system fine.
Going this route was cheaper than buying an NVR that was capable of running all the cameras at the resolutions of the cameras (my new cameras). I still run my old NVR feeding Blue Iris for those cameras.