I want to buy a used MacPro that I need for certain intel software. (have put off upgrading from my solid and well-working G5 for a long time). Don't care so much about it running 10.8 but do want it to still run 10.5 and 10.6 for software I want to keep. (maybe even a partition for 10.4). I may live in 10.7 perhaps??

Are there models to avoid, as well as ones that are a good choice?

I know the first couple generations will do this (if those were good machines), but I know their longevity will be shorter OS-wise. Also, are there other factors that I may not know about?