i would say get the xms 2, 2gb module to fit in slot 1 and 3. or 2 and 4.
that way you will have a dual channel 4gb.
3gb would be good if you had triple channel memory contrtoller, currently for core i7 series. that way 1gb X 3 would do.
xms2 some say is not better than value ram, but its like $15-20 expensive only. so get the xms 2gb X 2 dual channel kit!