Rather than using a double cog and derailleur setup find yourself a SA 5 speed hub. I picked up a NOS SA S5-2 Alloy hub that I used to convert a Kona cruiser to multispeed. Much cleaner looking installation IMO and a wider overall range than using cogs to half step a 3 speed hub. The S5-2 was even the right width to slide right into the Kona dropouts, about 118mm IIRC.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/