first of all, welcome to cocoontech!
let me preface this post by saying i'm a upb guy, and these opinions might seem biased that way, but also keep in mind that i'm one of the few people having serious upb problems (albeit, very occasionally). here's what i have gathered on the three main technologies that can be used with pre-existing wiring:
insteon - some people are happy, a lot are not. there seems to be a very high rate of switch failure and horror stories per capita with this technology. many on this board would recommend against it.
z-wave - more reliable than insteon... much fewer bad stories. however, if you were hoping to use your harmony remote for z-wave purposes (you don't exactly mention what model it is), the z-wave enabled remotes have not lived up to their hype (search for harmony on the forum for details if you were planning on going this direction). i can't be sure, but i don't think z-wave has any native IR control, so you would likely need a 3rd party solution to use your remote if it is IR only.
upb - as a general consensus, i believe more people are more content with this technology than the other two. of course, there will always be stories to the contrary on both sides, but i believe upb has proven more consistent anecdotally. on the down side, it is a little more expensive than the other two technologies. if your harmony remote is IR learning or web programmable, PCS makes a controller (either in-wall or tabletop, as you see fit) with a built-in IR receiver. your harmony could send the appropriate IR codes to control UPB links. add to that the appropriate lamp modules and some load-controllers (the PCS controller doesn't manage a load) and you should have your solution.
for your shopping needs, i personally recommend automated outlet. martin always has good prices and excellent service, both before and after a sale.
have fun automating!