Gavin,

I would say that in most cases leave soft affinity (OS Scheduler) in place especially with a SQL box. For example, on NT4 each NIC's activity is assigned to an individual (highest first) processor in an SMP scenario.

Use the System Monitor tool and gather metrics on key performance objects from the Processor and Threads section ie. Context Switches/sec, Thread States and Thread Wait Reasons.

HTH
Neil