It's like you're in the same kind of situation as the CPU one. Optimize HTTP, MySQL, scripts etc. or go for a RAM upgrade. You may install Opcode cache softwares like APC (from Pear) for PHP to make it perform better while decreasing the load.

3. Disk's all used (eh, I don't mean space)

Here you either have to go for a faster disk like SATA over normal IDE or SCSI over SATA. Well, I was just speaking generally. You have consider factors like RPM and cache to end up going for an upgrad that's worth it. The second option is to get multiple drives of the same class and spread the load across drives. One common methodology is to serve MySQL from a second drive.

IV) Conclusion

That wasn't of much help? My article might be flawed, ahh, excuse me. It's my first article and this thing really consumed quite a few braincells of mine. That's a bit personal isn't it? Let's get back to business.

FYI, In the example, the problem was with the I/O usage and hard disk getting slow.

A guide can never be complete in itself or offer you everything you will need to reach upto expert level (you need to keep learning to reach that level). Whenever in doubt, please DO hire experts to look over your server. Somehow, if you don't have the money to spend, you're stil safe! You can head to our Server optimization help section to get help with your server optimization.