And another recommendation for everything New Designz. Like QP said, with a delrin bolt, you need a heavier hammer to compensate for the lightness of the bolt. You can't go wrong with ND Slik shot ram, ND R.I.P valve, Rat hole hammer (not that heavy but recommended with the ND internals). As for delrin bolts, I would say any of the different sized ND bolts or the Eclipse Dart.
BTW, the ND internals are supposed to do wonders for your impy in terms of efficiency and consistency. Thats why I am ordering the valve in the next week to complete the set!
One last thing, if you haven't already, go onto
http://www.impulseownersgroup.com . You can find out loads of information about Impies and aftermarket parts on there.