paul haese basically stopped at 3x, and rob_t has done the odd 4x on the c9.25.
4x powermates are 2" and reallllly expensive, so a good 3x would be worth it
http://televue.com/engine/page.asp?ID=42
1/2 way down shows how the powermates change with distance
WELL WELL WELL
http://televue.com/engine/page.asp?ID=41
I say, go the 3x barlow, not powermate and with a range of extension tubes you can from 3x to 5x.
very handy for later in the year for smaller planets ie saturn or next mars or even neptune and uranus etc