Tom--
A little bit of Logic logic here:
Logic, like all 32-bit apps, can theoretically access 4GB of RAM. With 2GB installed, OSX is using 500(512)MB or more. That leaves you with 1.5GB or less for an app and plugins.
When you think about it, it's not a lot. At some point, you may want to max out your RAM. Keep in mind that you are using one of the most demanding DAWs with one of the most powerful virtual instruments on the market. Just a thought.
Here's what I recommend in the interim:
Run a disk utility like DiskWarrior. At least repair your permissions.
Eliminate the Digi interface from this test and use Built-In Audio. Unplug the Digi altogether.
Make a test project template with one VI instance and one standard patch loaded. Check the performance. Add one at a time until things start to choke. Compare that project size to what you were using before and you'll have a fairly basic but honest idea of what your system can handle.
You can try this another way by removing one instance at a time from your current project template to the same results, but given you are having troubles, it may be best to start with a clean slate and build up from there.
If the performance is a bit better, then I suspect one of two things:
1. If you are using FW hard drives and if the particular Digi interface you are using is running via FW, you could be experiencing FW logjam. This is not uncommon where FW drives and FW interfaces share a single FW bus (there's only one shared FW bus).
--Solution: get a card bus for FW or even change your HD setup to eSATA for better performance.
2. There could be a driver compatibility issue between Logic and Digi.
--Solutioin try a later driver. Sometimes an older driver can work better.
Apple has just released Pro Application Support 4.0, recommended for users of their Pro apps, including Logic. Check the link for more info:
http://www.apple.com/support/downloads/proapplicationsupport40.html