The [i]free[/i[ solution is to bounce the tracks with amplitube on and then use those instead of trying to process all of those in real time. You can then simply 'disable' the tracks that have all of the plugins on (I'd also recommend hiding them from the arrange window and the mixer to avoid confusion), so that if you need to revisit the amp settings later on, simply 'enable' the track again, fix what needs fixing and then bounce the track back out again. Admittedly this is very time consuming, and requires more patience and organisation than simply doing everything through RTAS plugins.
Also you could bus the guitars through the same plugin to reduce RAM usage (if all of the guitars are sharing a similar reverb patch for instance). This is a great way to free up your machine's resources at a stroke.
Other than that I can't really recommend any specific hardware, 'cos I'm a bit of a troglodite when it comes to recording - I prefer to move microphones than reach for EQ knobs. I have had some good results with the Sansamp PSA plugin (comes free with PT 8+), and I can run multiple instances without overloading the CPU.
Sorry I can't be of more help, but I've used the 'bounce workaround' on my old G4 powerbook (single 1GHz processor), and managed to mix a 20 track song for radio just fine.