If the new ram is the same brand & model, it should work with ram of similar specs. Oh I see. What are the rated voltages of both kits? Set the ram voltage to 1.9-2.2v. 5-5-5-18 should work. If not, then 5-5-5-15. If not, then auto & note the timings in bios. 4 sticks of ram needs more voltage. Finally, if vista loads, do super pi mod (google) 32mb. Then, do memtest (google) for 5+ passes.
FWIW, my ram consists of 2 different sets of specs & they run on more volt at 5-5-5-15.