This is just my opinion on boiling this topic down to an understandable summary level. I'm not an expert that will argue cfm, static pressure, velocity, etc. etc. etc.
When I was researching this, I found that there's so much information out there that it gets overwhelming in a hurry. Bill Pentz's website is amazing, but it's very detailed and can (quite frankly) be intimidating.
If you want "boy in a bubble" perfect air quality: 6" pipe, blast gates at each machine, cyclone, huge dust collector (aka… clearvue or similar), quality filter (Wynn) OR vent outside.
If you're willing to wear a respirator and still have to sweep up to some degree (but not much), and you're the typcial small hobby shop: 4" PVC, blast gates at each machine, thien baffle separator (google it for countless examples), 2 hp blower of some sort (and many will choose the harbor freight model based on price, the longstanding good reviews, and the fact that it looks like they came off the same factory run as the expensive models by different names), quality filter (Wynn) OR vent outside. In reality, I've been in a lot of shops with setups like this that are working just great.
Regarding pipe runs… avoid lots of 90 degree bends that kill air flow. Place the DC such that the runs are as short as possible.
Cyclone vs. thien baffle, and venting inside or outside. A cyclone or thien baffle is in place to separate out most dust and chips in an effort to save the filter from becoming clogged. Both work for this purpose, but the cyclone will work a bit better (for a lot more cost) but both are good solutions. However, if you're venting outside (rather than inside through a good filter) who cares? Save the money and build the baffle. The microscopic stuff will dissipate in the air anyways.
If you're venting outside, make sure there's a way for makeup air to get into your shop. Also, you'll hear about "sending your heated air out of the building"... Really, the thermal mass of the objects in the room will allow your space to heat up again in a hurry, so it's not that big of a deal. And, in most hobby shops, the DC isn't running for hours continuously. Me? I vent outside.
Keep the extra DC you bought, it can be used for a nice downdraft table.
Good luck! Again, I'm no expert, but hopefully a plain english summary was helpful.
edit: I've read TONS of info from dbhost (see post above that snuck in just ahead of mine).. and found all of it to be very reasonable and helpful. SO, THANKS DBHOST