Typically I just let Windows balance them out via the normal scheduling. If you want to limit, rather don't use processor affinity (and please don't set maxdop to 1).
You can set CPU ratios via a windows tool. Windows resource toolkit or something like that, I forget the exact name. That lets you set limits on CPU usage for various apps
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability