dedicated power line

I am in the process of having and electrician upgrade our home service to 200 amps. While he is at this it is quite easy for him to run a dedicated line for my hifi closet. What are the cost effective measures that i can have him put in place that have proven to upgrade sonic (and for that matter video) performance.

Make sure he wires them on the same phase leg. Otherwise, you might get a ground loop.
Not hospital grade, get better ones; it makes a major difference. I don't know exactly what you mean by 20 amp outlets, the current is carried in the wire and while a 20 amp breaker is different than a 15 the receptacles I use are labeled neither and while the female end of a 20 power cord differs from a 15 amp cord the male end is the same. So I assume they can be used on either, I have mine on both. They are much better built than ordinary ones. I use 2 20 amp lines and have used one for analogue and one for digital but have found I get better sound by putting my power amp and one powered sub on one and the other sub and everything else on the other. I use a PS Audio Quintet to isolate the digital and analogue equipment on that side. YMMV.
Depending on where you live, you may want to CONSIDER a whole-house lightning / surge protector. They are not expensive and may save you some grief down the line....and, during an upgrade as you suggest, easy to do.

While your electrician is at it, you may want to have him install a more robust ground. Since he has to disconnect the service to install a new 200 amp box anyway, a better ground can make a big improvement. You should have both circuits on the same leg and ideally opposite of your major appliances.