1. Crucial 2GB PC3-10600 DDR3 = Incompatible
2. Corsair XMS2 TWIN2X1024-6400C4 PC2-6400 = Should work
The reason I say should is because as much as I really like Corsair I'm not a fan of any of their XMS lines. Crucial's DDR2-667 also has the slower DDR2-533 and DDR2-400 'SPD' JEDEC information to run slower.
It's DDR3 and not DDR2 RAM, they are mutually incompatible and in addition they have a different DIMM slot design. Placing DDR2 RAM in an DDR3 Slot or visa versa won't fit and will cause damage. Further, the Dell MOBO is limited to 1GB/stick density and even if the Crucial was DDR2 it's 2GB/stick.
to make this simpler, do you have any good resources that explain ram speed and how it works, the basic 3 divisions i understand (ddr ddr2 and ddr3) but htne you have the speed and the buffered and unbuffered and all the other stuff