i dunno, seems like people are getting high VID 6600's and having to juice them up just to hit like 3.2.....
thirdshift and v-dub can do 4+ghz on theirs, but they are really an exception (and i think they have earlier chips). the q9450 will do 3.2-3.4 ghz easy as pie.
not a lot of differences. 10 onboard usb vs 6, i think the premium has one more pcie x 16 slot. probably a few raid-related things, maybe some SATA ports or something. probably nothing worth 50 bucks unless you have a crapload of usb devices to plug in the back.
here's a decent review by occ:
Asus P5Q Deluxe Review - Overclockers Club
i forgot to mention, if you are buying in the US, a q9550 is like, only a few dollars more than a 9450 on newegg. and it has another 1/2 multiplier, which "should" make it overclock better.
"nagasama no longer drinks tecate, or streaks out of the night for anything, and he has a fine wife so he has no use for your girlfriend...she is safe..."