I've just started the process of a massive overhaul of my current PC (which is not a horrible one, I'm just ****** off at the company I bought it from for all the issues I've had with the low-grade components they used to build it). While drooling over the thought of a pair of Radeon 5870s, the main concern around these cards is that they're developed to take advantage of a standard that isn't really in common usage at this point (DX10.1/11).
SE has said that they want FFXIV to take advantage of current technology or, to word it differently, that what constitutes a "good" PC now will be able to run the game but a "good" PC five years from now will be even better. Personally, I'm a visuals junky. There isn't a ton of opportunity to compare the difference between DX9 and DX11, but I did find this site that has some comparison screenshots and one tech demo gameplay video that highlights the difference. The devil is in the details, and DX11 is shaping up to add a noteworthy level of refinement to the current standard.
So do you care if FFXIV ships with DX10.1/11 support right out of the box? Is it something that you'd be indifferent to at release but would want to see implemented further on down the line, or are you happy with current high-end PC graphics performance and would rather SE focus their development attention elsewhere?