Well, running linpack on 2Gb (Vista eats 1GB of your 3) is of course way less efficient than running it on 5 or 11GB... but it should still be at least on par with prime if run at 100%.
Dualist (author of linx) said he discovered a way to force linpack running 8 threads in 1 instance even when there are only 4 physical cores, so we'll be rid of the dual instance run soon enough.