cent os. I do not know how heavily, we will hosting about 500 domains which are not a lot of havy sites, some forums, some smaller video sites. The problem is more backup and logs processing which is cpu intensive.
Hi. To be honest, for average accounts, 500 domains is not very many. If you have some high usage accounts you may have to bear that in mind. I would gather some statistics on existing hardware if these are already on some machine.
Additionally, to be honest, your CPU won't likely be your bottleneck. Your hard drives would be.
However, 500 domains is not enough to overload the server unless you have some really high load sites. Just check your bandwidth.
Just schedule your log processing, and such to happen off peak hours. Use your logfiles and stats to decide what's offpeak.
Also, you could execute the backup and log processing in low/idle CPU priority and look into LVM snapshots for the sake of at least OS level file consistency.
I would look at the motherboard first. Check tyan and supermicro for support and warranties. I use an identical spare board for my home pc. I changed it the other day to check my lan port. I would highly recommend getting an exact spare now if you want to use your setup for 5 years or more. Many boards aren't even available after 2-3 years. And don't forget amd opteron barcelona quad cores. The boards are widely available, and the cpus are at select venders like newegg. The 1.7 only uses 55 watts and goes for $214.