I would go with the GTX 460 1GB for a few reasons.
1. The WOW Cataclysm review on Tom's a month ago clearly indicated that WOW favors Nvidia video cards
2. The 1GB on the GTX 460 will certainly benefit your 1920x1080 resolution as opposed to the 768-meg version
3. Your power supply should be able to handle it. (you might need a PCI-E power adapter so double check)
4. You might be able to get a GTX 460 1GB for around $150.00
$154.99 after mail-in-rebate:http://www.newegg.com/Product/Product.aspx?Item=N82E16814162055