According to this, http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&dlc=en&docname=c03117539#N131,
you could go as high as a 8150, which probably won't be a huge increase that you'd notice over the 6100 unless your doing major rendering where the 2 extra cores would come into play.http://www.anandtech.com/bench/product/434?vs=699
They don't have a 6100 but a 6300 vs 8150, not a big increase to dump a $150 at.
As for cooling, it's hard to say if a aftermarket cooler would fit the HP board, give the caps around it enough clearence, etc, but the Hyper Evo 212 cooler is about the best cheapest cooler out there.
Also note, that motherboard only has a 970 chip on it and not the best power or VRM's, so overclocking on that board isn't the best nor are you going to get good results.