Adding RAM does help with the processing capabilities of your system. What it does is it makes it that your applications can access larger contiguous blocks of memory at a time, without your OS having to resort using swap space. Swap = bad. That means that your RAM usage is 100%, and your system will drop allocated memory to disk, so that it can still allocate memory for applications, but it is extremely slow compared with RAM access speeds.
Having higher RPM disks will make a difference as well. The comparison between 5400 RPM and 7200 RPM is enough to choose 7200 over 5400. I don't know about the 10K RPM SATAs. Usually a more imporatnt fact will be the Cache that it will have on board, and the connection type. If you're after ultimate speed, try Fibre Channel,Ultra-320 SCSI or Serial Attached SCSI. SCSI also has a very good track record for reliability and durability compared to consumer-level ATA and SATA.