Considering it's quite possible that you're not going to be using the A2000 for anything too serious using the 8UP card as Matt_H suggests and getting the extra 4 MB is the way to go.
Getting additional memory for the GVP 030 Combo is mega expensive. Is it worth the 3-4 x memory speed increase for the minimal times you go over 4MB? Depends on what you use you miggy for.
gertsy