those 2 gfx cards are on a par, depends if you prefer ATI or NVidia, or if youre thinking of adding a 2nd card later then depends on what the motherboard supports, more boards seem to support crossfire than SLI, though a lot of the X58 boards will do both.
Re AMD Id look at
this, its basically the 2 that kieves linked to
Edit: having looked at both the boards included Id recomend the 4870 for the crossfire option
As stated before if youre going to take the plunge today you'd be hard pressed to beat the i7 offering which is on this week only (runs out tomorrow) even by building yourself, its a steal at that price