Well, I wouldn't recommend a router at all. Runout on routers, while great for wood, is HUGE when doing circuits.
For example, my PC892 router (which is excellent) has a smallest collet size of 1/4 inch and PCB sized bits are 1/8th. I bought a DA300 collet to fit 1/8th bits (needed for circuit cutting) which fits into a 1/2" collet and the combined runout is +-0.01. That means that the smallest cut I can make will be at least 0.02" wide + the width of the cutter. Using a 45 degree V bit and cutting 0.004" deep that means the minimum cut width is 0.024".
DIP legs are 0.01" apart and pads are typically 0.06" in diameter, that leaves you 0.04" of copper between the legs. Its kind of hard to get even a single trace between those legs when you need 0.048" of space for the two isolating cuts!
I also tried some 1/8th adapters from Lee Vally in my 1/4" collet and they work a lot better (which is odd since they are just single split bushings really). Runout is 0.005". Still not great but you can do some fairly simply circuits with them.
If you are OK with swapping spindles for different jobs then I'd recommend getting a Wolfgang Engineering spindle specifically for PCBs. It runs at 24K RPM and has a max runout of 0.0004" (typical 0.0001"). The one I bought has no detectable runout at all with a 0.001" graduated dial micrometer.
Here's a video of me checking my runout
http://www.youtube.com/watch?v=nzSjIV9tr3E
The achievable results are fantastic and I have a short video of PCB cutting with it as well
http://www.youtube.com/watch?v=VUDW5pQbxAY
I'm working on a PCB routing primer video to help folks get started (not an expert's video since I'm no expert - but at least it shows an achievable way to get going). Dunno when it'll be ready as I work full time etc.
The Wolfgang Engineering products are available from
http://stores.ebay.ca/Wolfgang-Engineering
I have the TB-440 which I don't see for sale now. Its the TB-350 with an AC motor & mounts and it cost around $200 - $250 (can't remember exactly)
Just remember these are not built for high-torque but for high speed and exceptionally high accuracy. I can route 1/16th FR4 at 10IPM with a 0.063" diamond shaped carbide rasp all day with it but a 1/8th drill bit is a pretty much the limit with my setup. If course, a larger motor and better drive system (just using an "O" ring) would be more capable but I really don't see the need. I drill up to 0.06" holes and route out anything larger with the diamond shaped rasp. Those are usually mounting holes so there aren't many of them. Besides, the bits are tiny and you really can't put much more force on them without snapping the things.
Oh, here's some photos of what I've cut recently. The first and third ones are an experiment in surface mount chips. The traces are 0.01" and 0.005" cut 0.002" deep. I goofed on my Z adjustment and I think they're closer to 0.003" deep. The 0.01" traces came out great but the 0.005" got wiped out.
The second image is 0.014" traces on a board I have since soldered up and have working (a 4-20ma transmitter for testing actuators).