Measuring need for more CPU's

I have a 3rd party db, which I have little control over. ABout 250 users concurrent. Running perfmon my 4 cpus average about 49% for %processor time for each processor. My system queue length is about 1.4 I Have 8 gig of memory and will be going to win2003 and another 8 gig soon. Other than %processor time, any other good signs/alerts to monitor for needing another cpu. The box is an 8 way so I have slots for more processors

Those are the number to look at. I found that if the sql server is dedicated to specific app, 50% cpu time is not the problem. If multiple apps share same sql server, you'll start to see slow response.