If you are staying with a PIX platform and are running an appreciable amount of traffic through the PIX, I would probably stay in the 7.2(x) train - upgrading to the latest release in that train. 7.2 codes seems to run a little faster in that platform and has better memory management. If you require a particular feature in 8.0, if your firewall is not currently running a large amount of traffic, 8.0 should also run reasonably well.
If this addresses your question, please be sure to mark this thread as 'answered' for others' benefit.
Best Regards,
Kevin