You have two choices when selecting a computer IF you go the PC route (no, not MAC bashing!). One is to buy a system that already exists and the other is to build your own. To my knowledge you can't build your own MAC.
If you're building your own you already know enough about dual core processors, power supplies, RAID systems, etc. that you can figure it out from there. If you're looking for a pre-built system, you might want to start by searching around a bit for 'puters built for computer gamers. In some ways they're looking for the same things - fast processors, upgraded video, etc. You'll want as much storage space as you can get, though. Depending on what accessories you get, you'll have to decide if you want gigabit ethernet, integrated WIFI or Bluetooth, if firewire (400 or 800) is necessary, whether you want dual video cards, a built-in card reader. etc. etc. etc. The lis is (almost) endless. Keep in mind that the average computer is obsolete in about 3 years. Yes, there are older computers out there that are still being used, but they get left behind fairly quickly.
Different strokes for different folks. The 'net has a wealth of information on reviews, comparisons, etc. Let your fingers do the walking to start with.