It's hard to say, but placing 1000 clients on a single server is usually a bad idea, and you should split them to multiple servers.
A decent quad core with 8GB of RAM and 4 SATA-II drives in RAID-10 should be okay, but you'll need to go dual quad core, more RAM, and faster drives, depending on how much performance you want to offer to your users.
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Thank you guys, as CreativHost wrote to split clients on several servers ... how many "standard" accounts I can put on:
- Dual-core Intel 3040 (4 GB, 2x250GB SATA II)
- Quad-core Intel 3360 (4 GB, 2x500GB SATA II)
Splitting of the accounts would depend on what "application" your clients use - e.g. if you split the clients up as 400 / 600, and if 45 - 65% of the clients end up using something like WordPress / Joomla, etc on their accounts, then you may get a "bottleneck" ...
Also, if you have any resellers on your server, and if they are "overselling", then you would also have issues in the longer term.
The best thing to do is examine the installations on your server from the WHM panel to get an idea of it's current usage ... If you don't have any at the moment, then start off with a singler server, and then get more as and when you require more.
Which dedicated server will be fine for up to 400 clients and up to 1000 clients (HDD, Raid, CPU, RAM, bandwidth)?
Server should be Linux Cent OS, with cPanel, with unlimited addon domains, MySQL, PHP, 100 Mbps.
I wouldn't recommend stuffing a thousand clients on a single server. I'd buy a few servers and equally distribute the clients so that there would be less stress on the individual servers. I'd recommend starting with dual core servers (assuming that you're just starting). But it really depends on what sort of shared clients you'll be hosting.