I'm only on 7.2 but I have two WW1100E's running in HA mode supporting 2000+ users. You MUST upgrade the RAM - you need two more 1Gb ECC 5300F sticks to bring the system up to 4Gb of RAM. Not doing so will cause high CPU utilization as the system starts to leverage the swap file. I upgraded 16 other WW500E's with the extra RAM and the 7.2 upgrades went extremely well. I did the WW1100E upgrade to 7.2 and forgot to put tthe extra RAM in first and had serious issues with performance. However, since then they have been running fine with no problems.
The appliance is actually a Dell PowerEdge 860 so you can use 'off the shelf' RAM for that. It will run you less than $100 per system. That said, please don't take my word for this - open a ticket with McAfee support, verify the hardware (you can do a 'dmidecode' command from the Linux CLI to get the hardware details.) You need to make sure they're put into the right memory slots, you can look at the Dell site for the Hardware config guide.
The biggest issue is not hardware though - its the change in policy. I would HIGHLY recommend the following: install a 7.2 appliance ISO on a virtual machine. Use the Policy migration tool to copy your existing config over to the VM. Get the know how to manage the new version - it is quite different but MUCH better. Bring over your policies and your whitelists, then test test test. Once you're ready you can upgrade your 1100E and 'join' it to the virtual machine as another node or just export/import the conifg. (again, call TAC) Note that the 6.x -> 7.x upgrade requires a full reinstall.
You'll be happy once you're on 7.x - the McAfee guys have done an amazing job with it and keep adding great new features. Good Luck!