How many applications are you going to actually be running? How many resources does each app take?
Fred's listing is good but you might want to do just a little more homework first. As an example if you have an app that still uses 16 bit code in them you may want more processor since these apps like to hog processor power. Is the app memory intensive?
If you don't know then go larger. I would make sure you use at least a dual-core and would prefer 2 dual core processors. 4-8 GB of RAM will do you nicely. This will give you plenty of horsepower and room to grow.
The funny thing about Citrix is that once people see the power of the tool they start piling more and more things into it.
Hope this helps.