I have a few mods Mike.
Fuel mod: adjusts the A/F ratio in the open loop mapping, can be used to correct the lean mapping on a standard bike or with DNA 1 to 3 filters or all filters + pipes.
O2 controller: maps the closed loop circuit, most effective mod for low rpm control for 2007 onwards XT660's, eliminates most of the surging caused by the O2 sensors emission control mapping.
I would recommend both mods to map the bike correctly to get the best results, if money is a problem I would start with the O2 controller + twist grip mod for your surging.
Fuel mod
http://www.xt660.com/site/node/136
Fuel + O2 controller, can be bought by PM from a discounted Ebay price, or from EBay if you really want to pay more.
http://www.ebay.com.au/sch/7416kevin...p2047675.l2562
O2 controller same as above cheaper via PM.