Sizing of servers for a project

Hi everyone,

for an onlinecommunity project I would like to buy/lease own servers.

At the moment the project runs on a single rental root server. A 2CPU Dualcore Opteron 265 with 4GB Ram and 2x250GB SATA Raid1.
The project runs on Debian/PHP4/mySQL 4.1/Apache2.
The maximun users same time online are 750-950 (in the evening hours). The performance of the server is ok at this oint but I guess the maximum concurrent users are nearly at maximum. Because the provider offers no possiblities to upgrade or to add a second dedicated server for the database I would like to get own hardware and own colocation.

I’m not sure about the sizing of the servers. In the next year I expect the online users to grow up to 3000 concurrent online.
If these goal will be reached there should be no problems to extend the hardware but at the moment the budget is limited.

Would it be better to get one big server for the apache and one for the database? Or would it be better to get maybe 4 or 6 cheaper single CPU machines and build kind of cluster?

Maybe for the database something like that:
Dual-DualcoreCPU machine (I prefer the new Xeon CoreDuo)
SCSI 15000rpm HardwareRaid 10 with 4 or 6 drives

and maybe the same for the Apache but not that expensive harddrives. I think Raid1 is enough.

These machines could then be extended with RAM up to 32GB if needed.

What are your thoughts?