Hi,
You're traversing a few major changes there.
I can't advise about Avid's cross/upgrade deals etc, but I can tell you that the version you upgrade to is likely to be dictated by the system you're on.
Are you sticking with the G5? What operating system can that take you to? 10.5...possibly 10.6?
Check out avid.com for their compatibility tables. That should set you on the right path.
There are other things to take into account too. There are usually amnesty options...for example, if you buy PT10 retail today avid will send you an 11 license.
That may not be true right now; I don't know the details, but it'd be worth finding out, huh?
Also, have a look at the release notes for each major version.
PT 8 brought in elastic pitch, air plugins, track comping, automation lanes and some good V.Instruments.
Pt 9 dropped the hardware tie so you can use whatever interface you want. It also brought in ADC!!<whoop>
PT10 brought in clip gain and, I think, AAX support.
PT11 is 64 bit AAX only...No RTAS, but you get offline bounce!
Because of the dropped hardware tie, 9 and 10 require and ilok1+license. 11 requires an ilok 2+license.