[pablotec]

For the diference of prices I think that the best option is to buy two Process card for your system, actually only few plugins are exclusively for Accel card (such as Revive).
Respect to power, Accel can charge up 80% more plugins than Process, it´s true, but in the real use is about 40-60% more power, because in the assigments of voices, delay compensation and many plugins there is not diference in both cards (except Accel acepts more voices).

One Accel card is about 2500$, but you can find Process card by 600$ each (you can find on AltoMusic, NY, official dealer of Digidesign).

Your G5 can accept 3 PCI or PCI-X cards (depends of model, single CPU is PCI, and double CPU is PCI-X).
PCI-X accepts 64bit data at 33/66 or 100Mhz
PCI only accepts 32bit and 33Mhz (the conector is smaller)

Both Digidesign cards works with PCI or PCI-X, but you can obtain more bandwith and voices with PCI-X
Your Mac do not accept PCI-e format!!

With this configuration, you will obtain a HD3 Process machine, 27DSP for your mixing (for DAEengine, voices, delay compensation and plugins TDM).

Regards