If you were looking to upgrade the fork on the Boulder, may as well look at the frame and all the components as well and upgrade them, as it is a pretty basic bike.

If you must upgrade the fork, would look at a 2012 Rock Shox SID RCT3, or if you can find a NOS 2011 Rock Shox SID XX World Cup Carbon, that would be nicer, getting a good fork for a tapered headtube frame is getting harder now.