Andy,
I just spent the past two weeks relocating machines and revamping the dust collection ducting in my shop. Contrary to all the great advice Bill Pentz offers on his site, I've found that there are much more economical ways of doing it with excellent results. I used the 4" ID sewer pipe from HD at under $8 a 10' stick and it only took 4 sticks plus a few fittings for the whole setup. The key is good grounding to minimize the static to virtually zero. I do have a 2hp collector with the filter canister from Grizzly and additional air filtration in the shop so I'm with Bob that you might want to think upgrade on your DC. Another great support to maintain good suction should you use less than a 3hp DC unit is to make sure you have good blast gates to cut off each machine at the main line, and keep the shortest possible runs. Go with the rigid lines all the way to each machine if possible and use the flex ducts to create long radius elbows where you need instead of regular elbows. My system splits off to each side of the machine area (unorthodox I know but it works well) so I also have gates to isolate the side of the room I'll be using at the time. I reworked my entire system for under $150. Having so many blast gates is not the most ideal setup because you have to consciously make sure and close or open only the ones you need for the specific machine you'll be operating at the time. But, in a small one man shop on a tight budget, it works just fine for me because it doesn't take that much time and is essentially second nature to me now. I currently have the best suction I've ever had in my little 460 sqft shop here at home since I set it up and couldn't be more pleased. Again, grounding is extremely important if you go with plastic pipe because you don't want to get zapped by static while your hand is in the vicinity of a moving sharp object.
In regard to grounding, I've found that it's not necessary to run wire through the pipe (which can cause clogs), but rather, I add 3/4" lath screws piercing through to the inside of the pipes at about 8" intervals on either side staggering so that actual exposure in the pipe is about every 4". Then I wrap the exterior of the pipe with bare copper wire in a long spiral fashion connecting all the screws for the length of all the runs and grounding to the machine at the end of each run. Don't forget to ground to the DC as well. I can post a few pics of it if you're interested, and if not, well, that's what I did and it works great for me. The shop is virtually dust free now when resawing exotic billets, or bolts of spruce. 
), there was a study done on the screw through the pipe method and the purpose was to see if indeed it would eliminate static inside the pipe when using the method I described. This is the reason I switched to this method because it virtually eliminates clogging if designed according to my description and used with an adequate layout and DC. They found that the static discharge distance in an evac system is an average of 4-3/8". Thus, the every 4" coverage in the pipe. I've been at this (woodworking) for over 35 years much of it professionally and in a large shop environment, absolutely metal is the way to go. 
Another big culprit for clogs is moving the dust and chips at less than 800 cfm. A large bandsaw needs a minimum of 800 cfm to move the waste efficiently and a 12" thickness planer needs around 600 cfm with a table saw at about 400 cfm. These are minimum for efficiency and to help prevent clogging. The longer the runs you have and the more turns, the less efficient the line becomes. Thus the need for better isolation in the typical small shop using one machine at a time.