I run PT8 on a 2.5 quad core G5 with a BLA002. I have had it for a few years now and it runs great. No issues with PT8. It's used for professional sessions at my home studio and it gets a good workout. If I am running 48k or 44.1k (I usually never start sessions at 96k) I have no issues mixing with maximum tracks. I usually don't have issues with using too many plugins either.
All that being said, the Intel equivalent does run faster and will continue being supported in the future. Although I have serious doubts that snow leopard will be Intel only. If the price is cheap, I personally would buy it and spend the savings on other gear. The added RAM and Vid card is nice, but in no way should affect your decision (or the price), because you don't need either. 4-8 Gigs of RAM is fine.
Hope this helps, email me if you have any questions.
adaM
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