Xps 420 quad channel ram

I don't think so, it's quad channel and the XPS supports only DUAL channel memory.
Since it's an OEM rig, it could reject the RAM altogether.
Also depends on what config of the RAM are you taking since it's available in Kits.....
"Available in 4GB(1GBx4), 8GB(2GBx4) & 16GB (4GBx4) Quad Channel Package"
So what sort of a combo is going to work on that Dell mobo is really difficult to say.
 

xps420_64

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Ok thank you for the reply.
i was thinking of 8GB 2GBx4 PC2-6400 800Mhz C5 DDR2 Black Dragon Quad Channel

but I'll stick to the parts dell recommends