This thread will answer many of your questions:
http://www.iceinspace.com.au/forum/s...ad.php?t=35142
In terms of Powermates, you're mostly going to use either a 2x or 2.5x with your C8...since a 4x is going to be unusable on all but the very best of nights.
I'd be more inclined to consider a 3x Televue barlow for planetary imaging with a C8.
But if it has to be a Powermate, you might want to check Televue's website to check out how they perform.