the channel wattage refers to how much power the each individual knob (Rheostat) can handle. a typical 120mm fan is about 4-8 watts. larger fans (200mm+) are around 8-15 watts. as you slow the fans down the excess power is absorbed and dissipated by the Rheostat. in most of my builds, I tie like fan model and Purpose fans all on one channel. example:
2 x NZXT 120 mm intake front fans - channel 1
1 x NZXT 200mm side intake fan - Channel 2
1 x 120 mm rear exhaust fan - Channel 3
2 x 140 mm top exhaust fans - channel 4
6 fans 4 channels using a Scythe Kaze Master Pro 5.25 Fan Controller 6 x 12watts channels ( I have 2 spare channels )
to put more than 1 fan per channel you will need a 3 pin "Y" splitter cable.
ZALMAN, NZXT and Scythe are good Fan controllers, I have built with those several times. 1ST PC CORP seems to get good reviews also, but I haven't built with one of theirs before
you want 10 - 12 watts per channel to do 2 x 80-140mm fans on each. following the above example you only need a 4 channel controller.
these are all good picks 4-6 Channels:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811997046
http://www.newegg.com/Product/Product.aspx?Item=N82E16811998077
http://www.newegg.com/Product/Product.aspx?Item=N82E16811999171
http://www.newegg.com/Product/Product.aspx?Item=N82E16811997019
http://www.newegg.com/Product/Product.aspx?Item=N82E16811992007
3 pin Y splitters
http://www.newegg.com/Product/Product.aspx?Item=N82E16812189063