Moving from 7.4 to 8 really doesn't buy you much, other than the Creative Collection(MiniGrand,BOOM!, DB-33, etc). Pretty sure there was a version 9 for M-Powered, but there's really no point in going for that. All the M-Powered versions limit you to M-Audio interfaces, while the regular Pro Tools can use those as well as almost everything else under the sun. Then, with the exception of the ProFire 2626, the rest of the M-Audio interfaces from that era were built to be cheap, not good(can't speak for the current crop of hardware, but my advice would be to stay with the bigger players like Presonus, Focusrite, RME, etc).
Back to the software, if you are going for an upgrade, move to PT 11(or newer). That gets you access to all the current crop of AAX64 plugins and VI's. That's an upgrade worth doing