Performance of a Poweredge 840

What I am asking for is how to squeeze blood out of a turnip...

I have this 840, is does TOO  much. It is a DC, File Server, has Quickbooks running on it.

Whenever I do the Performance Analysis is always tells me "The system is experiencing excessive paging." Always. I know the culprit. We are phasing Quickbooks out.

But in the meantime, any of you Guru's out there that can tell me is there ANYWAY to to increase the performance of the disk subsystem? Without buying new drives. "Tweak" the disk subsystem some?

(Yes I have maxed out the memory. 8 Gig.)

• It has a PERC 5/i Adapter. 8x. Firmware 5.2.2-0072. Driver: 2.24.00.64. Cache 256 MB.
• 3 HDs all in RAID 5 config.
  • All 465.25 GB. Two WD other Seagate. 

Thanks,