Looking at the Supported Guest and Host Operating Systems in the Compatibility Guides it shows that last supported 2.x version is 2.0.6 under Mac OS X 10.6.1 however assuming you'd be running the latest Mac OS X 10.6.8 it show 4.0 as the oldest version supported. So obviously upgrading to VMware Fusion 5 is what you'd probably end up doing if you upgrade to Mac OS X 10.6.8.
That said, you's still be able to use your existing Virtual Machines although for best performance the virtual hardware version should be allowed to be updated by VMware Fusion 5.x when first run. Also be sure to make proper* backups of your Virtual Machine's before doing anything.
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* It is a known fact that Time Machine is not 100% reliable backing up/restoring Virtual Machines under all circumstances/conditions. Also backing up Virtual Machines via Time Machine is disk/time intensive and wastes a tremendous amount of space for something that may be corrupt and worthless come time to restore it. At a minimum I would exclude Virtual Machines from Time Machine and with the Virtual Machines shutdown, not suspended, and VMware Fusion closed then manually copy the Virtual Machines Package(s) to an alternate location, preferably on to a different physical hard disk. Then keep the User Data that is stored within the Virtual Machine backed up off of the Virtual Machine on a regular basis so as to always have a current User Data Backup. If you have to restore a properly backed up Virtual Machine that is not as current at least you'll have a working Virtual Machine and current User Data to go forward with when you find out your Time Machine Backup of the Virtual Machine fails.
Also have a look at: Best Practices for virtual machine backup (programs and data) in VMware Fusion (1013628)
Message was edited by: WoodyZ - Added VMware KB link.