No, it's not always faster, but may be faster in some type of operations.
Regarding your config:
Dual xeon 2.8 , 2gram , 73G SCSI 10K RPM vs 2 x Alton XP 2.6 1G ram 120G 8mb IDE, I'd choose the Dual Xeon. Reasons:
1. Instead of having 1GB in a server running apache, and 1GB in a server running SQL, you'll have 2GB for both. This may look the same, but it's not.
2. A Dual Xeon 2.8 may support a heavier load than 2 Athlon XP 2.6, as you can share the load of SQL and apache between the processors (also consider hyperthreading), what won't happen in different servers.
3. SCSI is much better than IDE for disk intensive applications as sql, and is more reliable.