well, if i had one lying around, i would research if it were possible to a second processor via Add New Hardware or something at support.microsoft.com
i would sure system files backup first and i would handle that first procesor with great care...i probably wouldn't have stomach for it at all actually...
you sure the processor is the (only) bottleneck? you can shotgun this thing all day, better to pin it down for sure.
maybe double the ram again. oops, ahem. i love shotgunning. ram less dangerous to attempt than processor imho. i am chicken.
you may need to adjust or 'tune' pacware db engine config (which begs question of: what engine?) to use the additional ram. see mfg website for performance tuning. what is happening in the afternoon? remote logins? heavy reporting? heavy data entry by lots of simultaneous users? antivirus updates? windows updates running? backups beginning? database maintenance? defragging? is the stuff going on inside or outside lan? you sure the server is the bottleneck. anything else working hard? they complain of slightly slower what? login response? db response? is that when lots of db access and activity? anything in event log?
turn off all unnecessary services. see support.microsoft.com for what you need to leave on. what processes/services/applications exactly are running during this time of high processor utilization. what is in event logs at that time. plot events vs time. tune all for performance carefully based on research at support.microsoft.com.