I did quite a bit of research into memory capacity of the Dell Precision M4300 laptops.  We use those as well as Lattitude D830s here at Headspring.  The chipset is the same, just the case is a bit different (and video card).  Folks have had mixed results on moving to 8GB of ram, so I decided to purchase a single 4GB chip of PC2-6400 800 Mhz.  I popped it in slot B and worked like a charm.