Man i agree with chad outten
like he said 50 per 1gb of memory
but look for some accelerators like memcached or eaccelerator (donno if the two of them will cause a bug), i was using eaccelerator to see the benefits of it but donno the answer yet.
Will be good to put some fast hard drives like SCSI OR SAS about 10k-15K RPM makes the server fast uses too some raid 0 with raid 1 (about 4 hard drives 2 for raid 0 and more 2 for raid 1 (mirroring))
Can you put bellow your apache configuration about modules you are using and mysql variables??