First off you can't get the PWS 390 with a Xeon CPU at all. You would have to move up to the 490 or 690 both of which are dual socket Xeon systems and cost a lot more than the 390. What do you do with the PC? The 390 is Dell's entry level workstation and has the Intel 975x chipset and uses ECC RAM and pro graphics cards. But to answer the question no difference. The 490 and 690 use the Intel 5000 based workstation chipset and Fully Buffered DDR2 ECC which has really high latencies compared to the 975x memory access so even though most quad Xeons are on a 1333MHz FSB the difference in real performance due to RAM differences is not much comparing same amount of RAM and close to same speed CPU's and cores.