Greater lift and greater duration are two different things, so need answering seperately really.
Not something I profess to be an expert on, cams are a very black art, but I will try and answer from what little ive learnt over the years from my relatively limited meddling with different cams.
More duration in principle should lead to more power at the top end but gives you a problem of where you choose to time the cam and hence where you acutally have that extra duration in the cycle, too much overlap with the exhaust cam can result in blowing the mixture straight through the engine at low rpm, but time it out the other way so it starts later and you can end up with a situation where its not actually open at the right time in the cylce and potentially in an extreme case can result in the cam open too long when the piston starts to come back up, which means effectively a drop in VE due to the mixture coming back out again, and hence effectively a drop in dynamic CR (this will even show up on a compression test of the engine).
More lift without extra duration (although that will depend on where its measured of course!) will have a less negative effect at low RPM typically and can still yield good gains higher up but the problem then is massively increased strain on the valve train as it results in greater valve acceleration rates, so for really big lift short duration cams you need to be moving over to a proper set of lifters instead of the hydaulics.
To answer your next question that you havent asked yet but no doubt are about to, the standard valves are FAR more than big enough, and there is no point changing them until you are talking absolutely huge power levels IMHO.