Did you look at the whisper gnome as a race? Great benefits with no level hit. I also ran this class in a campaign. The most effective combat combo i used was improved invisibility mixed with bow feats(ie. rapid shot, point blank) and the sneak attack damage. Add cats grace and other boosting spells to taste. I found that spells should be used to supplement staying invisible and boosting your odds of hitting and doing more damage, not being your primary means of damage. Any spell that keeps you out of HtH combat while dealing sneak attack damage should be your primary concern. Also, a high sleight of hand with improved invis. will give you options with ranged legerdemain in combat you never thought of. See a wand on an enemy wizard you'd rather he not have? That healing potion you need on the clerics belt. the list goes on. Also, hide with a whisper gnome is a very disgusting skill. My whisper gnome had a 25 hide check at 12th level. Yes, the bonusess stack.