I have 250 users, currently with unlimited storage, though only a few are using more than 1gb. I went with a Dell 2950 III with Dual Xeon 5400, 8gb ram, 6x 146gb SAS, last year and it has been plenty of power. Since then I have removed 2 of the 146 and added 2 1tb drives for an HSM. Down the road once storage drops in price, we'll probably upgrade and revert back to 6x 600gb SAS drives for simplicity.
__________________ Culley Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now |