Further expanding on what brett said above, let me supply some numbers.
As GTX570/580 are fairly new, there is not much real world data out there and so let's settle for GTX470/480 for the moment.
for GTX480 SLI + 9800GT (Physx)
for GTX480 SLI + GTX285 (Physx)
These links are indicative (especially the first one) to show that even a humble 9800GT can help out by being a Dedicated Physx when paired with GTX480 SLI (confirmed by 2 individucals- rhcliff12 and jdookie). The key is ''resolution and AA''
. At lower resolution/AA (eg 1920x1200 + 4AA), a pair of GTX480s is obviously powerful enough to do both rendering and Physx. At higher resolution/AA, the benefit of a 9800GT Dedicated Physx card begins to show.
And, as for PCIe lane speed assignment goes, your EVGA X58 FTW3 will go from ''x16,x16'' to ''x16,x8,x8'' and I think this reduction in speed is negligible (ie worthy to exchange for the benefit of a Dedicated Physx card as GTS450) at around 2%.
Then, there is the heat issue. For OP, the upper GTX570 sits in PCIex16_1 and is unaffected. But the lower GTX570 (in PCIex16_2) will see a rise in temperature as its air intake will be disrupted to a certain degree by the addition of a Dedicated Physx card (at PCIe x16_3). But it should be manageable.
And power-wise, OP's 1200W has no problem at all.
So, all lights go green, IMHO.