Have any you conidered a Sram dual drive hub to give you 27 gears rather than going for more chain rings and fitting a front derailier? It would mean getting the hub built into a rim. I did this with my Mezzo folding bike and I am very happy with the results.
Absolutely a great way to go, except it's not cheap. It will give you better gear range as well as better spacing between gears (if you want it).