First off, have a good time building what ever you decide on!
Check out the review sites and the bench marking sites.
You may not need the fasted unit out there, but something in the top 10% for speed will likely extend the usable life a couple years ( and that is about the point where the processor proce takes a big drop.).
Two big factors are how much money you want to spend and what all do you want to do with it.
You can get up to $30K into a extremely high end Xeon workstation that is fully loaded.(Cad and animation mostly)
or I saw a Atom netbook at Costco a while back for $175 that had a external video and usb connections which could make it suitable as a home / portable system. ( Netbook might be a bit light for video editig. Almost bought it for my wife, but she wanted to do some videos for the kids.)
With the more modern systems, any system less than about 3 generations old will handle your current needs.
As far as future needs, Intel TICK TOCK release strategy favors motherboard being capable of supporting at least 2 generations of processors.
I would start with what you plan to do with the system rather than any component of the system.
So breaking it down.
The type of PC I want to build is for home/business use.
This usually implies a windows system
Since you want it for a few years, Windows 7 http://windows.microsoft.com/en-US/windows7/products/system-requirements
Minimum -- More is better! I would beat these by 2x or more if possiable.
Additional requirements to use certain features:
Internet access (fees may apply) (Wired and wireless NICS)
Depending on resolution, video playback may require additional memory and advanced graphics hardware ( Video capture or play back cards)
For some Windows Media Center functionality a TV tuner and additional hardware may be required ( another coption card or device)
HomeGroup requires a network and PCs running Windows 7
DVD/CD authoring requires a compatible optical drive (CD / DVD . Blu-Ray burner?)
BitLockerrequires Trusted Platform Module (TPM) 1.2 ( hardware board feature to look for if yo want ot use this. Not many boards have it)
BitLocker To Go requires a USB flash drive
Windows XP Mode requires an additional 1 GB of RAM and an additional 15 GB of available hard disk space.
Music and sound require audio output ( audio down or as a add in card again)
I work for an insurance company so I do a lot of remote logins
The faster the network the better (GigBit prefered), (assuming it fast on both ends or might be some day. Your home Internet connection will likely not get faster than 100Mb (most are 10Mb) unless you have a home network where you share files or stream from one system to another 100mb is good,)
as well spreadsheet/database work.
Need a little bit of HDD space, A SSD speed would likely be a Plus factor.
If you want the best in reliability, a RAID and 2 or 3 SSD
From the home side just normal web browsing
, video editing
Ouch, you just raised the stakes,
Much larger HDD storage if you plan to leave the video on the computer.
Better video card and video capture card.
Video editing is second only to CAD \ animation for computer intransitiveness (is that a word?)
the biggest ect.ect. is if you want to run addtional add in cards, how many pci slots do you need.
Good luck to you!