Re: XPS 420: RAM Frequency is 667 MHz instead of 800 MHz.
I'm very curious what you ended up doing with your XPS 420 . . . I'm just now discovering this problem. I was orignally running:
STOCK - 2 x 1GB PC6400 800mhz
STOCK - 2 x 512MB PC6400 800mhz
This gave me 3GB at 800mhz verified in BIOS, but when I went to drop in G.Skill 2 x 2GB PC6400 800mhz with a Latency of 5, along with the STOCK - 2 x 1GB PC6400 800mhz I was shocked to see 667mhz in my BIOS settings. Sounds like the SAME ISSUE. Unfortunetly this maybe my last Dell as it should be everyone else in this post, they made their last excuse. I spent and hour with an XPS tech trying to explain this to him. He only proceeded to tell me though its not impossible in most likely improbable and that was that. This motherboards eSata port also gave me hell, though they fixed the issue, it is by far perfect as it takes twice as long to boot the controller.
Did you ever purchase the secound identical RAM set, and if so, did it work? I really don't want to buy $65 of 800mhz RAM if I'm only going to get 667mhz out of the XPS motherboard.