Dedicated Servers - Sizing?

[interfx]

I currently have 2 reseller accounts (1 with HostGator, and 1 with other company). I have clients that run in total 5-10 Vbulletin (MySQL) sites that are quite active (200-300 users online at once)...

How do I go about sizing a Dedicated Server? How do I know what size to get? I'm on a limited budget, but wanted to ask how do you "size" a dedicated server? I'm very happy with HostGator, but find my server loads around 5-6, and would like to concentrate all of my accounts to one place, one server, one support...

ie.

2.4 Ghz Super Celeron ($174)

vs.
2.8 Ghz Pentium 4 ($219)
vs.

Dual 2.4 Ghz Xeons ($374) - Too Expensive for my budget

What should I be looking for from my current account to understand better what I should be looking at? Also with dedicated servers, does it have automated billing scripts?

Thanks in advance -
InterFX

[TeeJa]

I would at least go with the 2.8 Ghz Pentium 4 ($219) or contact sales when they will have the "Elite".
I have the $174.00 which is fine for me now, but know in time I will have to upgrade and I don't have 300 users at once on any of my boards, but a lot of traffic on the other accounts from visitors and Mysql access. One peron sends out a news letter every week to 1600 and it almost stops the server, lucky he does it at 5AM, slow time.
If you go with the $219 now, you can always upgrade to the next and have support copy your accounts/boards over.