I think that power conditioners improve things for components with inferior power supplies, like cheap dvd players. High end components usually have top-notch power supplies that should get rid of most noise on the A/C line.
Power conditioners can help stop cross contamination by isolating everything. I have yet to see any real world benchmarks (not by the manufacturer) and graphs that show differences in sound quality. I thought I heard (not measured) better bass frequencies when I hooked everything into my Belkin PF31, but then again it could be psychoacoustics. I'd like to do some actual measurements some day.
Belkin has some nice products in their PureAV line up. The PF60 looks pretty nice. The Hybrid AVU1500 looks even nicer but has a pretty hefty price tag. CDW in Canada has it for $815.99 which is pretty good.
http://www.cdw.ca/shop/products/defa...px?EDC=1362179