robbins

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montyuk

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its the sheer level of bandwidth throughput that that particular slot is capable of.

so in plain english, the x16 is eight times faster than the x2

so just put them in the x16 slot and you can put in pretty much anything.
 
OP: Nice PSU, quality choice.
Just to add to montyuk, a X2 slot is much smaller than a X16 and practically all PCI-E cards are designed to fit the larger X16 slot.
Yes, a HD4870 will fit your MB slot but make sure it`ll fit your case, its 10.5" long!