yes the 4890 was kick ass in it's day but is a little dated for the rest of your system (about 1-2 years behind the rest)
Crosshair IV means you can run multiple cards are are NOT limited to a Dual GPU Card.
i would consider the 7970 and look at getting another later down the track.
Tigerdirect.ca has an XFX 7970 3gb for $579
http://www.tigerdirect.ca/applications/SearchTools/item-details.asp?EdpNo=1814879
saving you over $200 and BLOWING away ur 4890 & Beating the 560TI 2WIN by a decent margin.
also the GTX5xx series is due to be replaced soon with the release of GTX6xx where as the ATI 7xxx series is still very new and current so finding a 2nd one in 6 months time is HIGHLY likley.