Our advanced hosting solution at RackSpace consists of a firewall in front of a Cisco load balancer unit. We have 5 web servers which are Dell PowerEdge 2650 boxes with 2 GIG of ram, dual 2.0 GHz Intel Xeon processors and 3 15000 RPM SCSI drives running Raid 5. Each site hosted connects to one of 3 seperate database servers over a backup network. Each database server is a Dell PowerEdge 2650 server with 4 GB ram, dual 2.0 GHz Intel Xeon processors with 3 15000 RPM SCSI drives running in a raid 5 configuration.
Anyway's you say you have it running on a P4. I would get a Dual xeon with 2gb of ram. But if you need alot of room to grow go with two dual xeons 2gb ram and one of database one for httpd. It is costly. Scsi is also an investment
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How large is your forum? But since you have notice lag with it, it's probably time to get another server. Your options are probably either migrate to a single dual xeon server with 2GB ram and perhaps SCSI harddisks, or you can get another comparable server and farm the SQL to it's own server. Both should work fine.