When I ran Quickbooks I found I got better performance by having the app running on the same server as the database and not running locally on the client machine. Although you may get a slow inital program load time of 2-3 minutes, once the db was open we were able to navigate records much more quickly.
and yes, it does suck for your purpose, one of my resume skills around that time was "migrated finance dept. to SAP from quickbooks based accounting system"
You may find the switch to gigabit worthwhile. With Cat 5e you will see a performance increase if you have gigabit ethernet cards on the clients and network gear. I use 5e cabling on a ghostcast server with a gigabit switch and gigabit clients connected directly to it. I saw a nice speed increase in casting times once I used the gigabit switch over an older 10/100.
Cat5e is rated to 350MB, Cat5 to 100. I think Cat6 and 6e are 550 and 1000 respectively but I could be wrong. 6 isn't even a standard yet...i'm not sure why there is such a long delay in that process.