I had an XPS 400 for about 2 years before I gave it to my daughter and I ran 4x1GB Corsair XMS2 Pro for about a half a year. Worked great for me anyway. The only issue will be the RAM speed...the motherboard that comes in the XPS 400 can only run RAM @ 667MHz and this can not be adjusted in the BIOS.
It might be a waste of money to put into a XPS 400...any value RAM will be fine.