Outlook White screens and slow performance

Hello all,
We have a customer that continues to have problems with Outlook performance. They get white screens and spinning hour glasses much of the time. They are heavy Outlook users and also use Teamscope CRM. Every piece of equipment has been upgraded and it got better for awhile but now they are having problems again. Running scanost seems to improve things for a few days then it degrades again. Several users do have large OST files in the 4 GB range. Public folders are currently 18GB and Private 24 GB.
I keep the exchange databases defragged and everything is patched. Currently both exchange databases reside on the same raid array. Should I consider splitting these between disks? Teamscope stores everything in public folders.

Before the upgrade they were running SBS 2003 and had similar issues. The only thing we brought over from the old system is Teamscope CRM. Does anyone have similar issues with this program?

Other than this issue you can sit at any workstation and the network, internet and Office are rocking fast. We can move huge graphic files across the network with no problems.

Any suggestions would be greatly appreciated!

Here is the system information:

Server:
Dell Poweredge 2900 III
SBS 2008
Dual E5420 2.5GHz quad core
8 GB Ram
2 - 300 GB SAS (raid 1) OS
2 - 500 GB SAS (raid 1) Data
2 - 146 GB 10K SAS (raid 1) Exchange databases priv and pub

Workstation:
All are new dual or quad core procs
4 GB ram
Sata hard drives
Windows 7 32 or 64 bit
Office 2007 or 2003

Network:
Dell gigabyte switch