From what I can see the PCI-E x4 slot on your board is PCI-E 1.0. So 250 MB/s per lane x4 = 1000 MB/s. You should have plenty of bandwidth but is the ASUS card backwards compatible to PCI-E 1.0? The specs say nothing as far as see. My educated guess would be it would work just fine. If you are running a single GPU you could always use the 2nd PCI-E x16 2.0 slot if need be.
My 2 cents
Bill
Actually Bill, looking at it again and I think that you are indeed right. As long as the bandwidth wasn't too much impaired by the graphics use in theory it should be ok. I apologise avinitski for my initial thoughts on the subject and I can't really see a problem with the compatibility of the card.
I think this quote from tomshardware.com says it all, just go x58. "Bandwidth limitations on Intel's side will require a chipset refresh. It's possible to manage existing bandwidth more efficiently through PCI Express switching, though, which leads us to our main recommendation. If you're truly concerned with buying a board now that includes USB 3.0 and and SATA 6Gb/s support, it makes sense to purchase a higher-end platform in order to make sure you get a configuration with switching capabilities, at least (assuming, of course, that you're in the market for an H55-, H57-, or P55-based setup). Should you instead opt for an X58-based machine, these potential bottlenecks won't be an issue. Of course, that's a viable option as well and, frankly, more attractive to performance enthusiasts."
http://www.tomshardware.com/reviews/usb-3.0-sata-6gb,2583-8.html