Performance...

[StupidGlitch]

Processor : PROCASE PENTIUM 4 3.0/3.2 GhZ HT
Memory : 1GB DDR2 RAM

Processor : Intel DualCore E2200
Memory : 2GB DDR2 RAM

Processor : AMD64 X2 4450E
Memory 1GB DDR2 RAM

What better perfomance could i expect say from the bottom 2 over the top one?

Also how many users roughly ( including guests ) would they be able to handle?

Won't have more than 5-10 sites on, 1/3 to start off with.

 

[Tomasis]

Probably best performance will be with Intel E2200.
Not far away will be AMD Athlon 4450E
And far away will be pentium
Performance between E2200 and AMD 4450E will be quite similar, so choose the cheaper one

 

[Viz0n]

You will want to 2GB RAM at least. 1GB ram will give performance issues if you plan to run CPanel and want to Cache for faster performance(page loading).

If I had to pick between those 3 servers, I would go with:
Processor : Intel DualCore E2200
Memory : 2GB DDR2 RAM

Also how many users roughly ( including guests ) would they be able to handle?
If you properly configure the server and services, a lot.