I don't think that they have drivers supporting XP and it's a power hungry card.On ati's website it recommends a minimum of a 750W power supply but it's an awesome card here take a look at the comparison with a hd 7970 http://www.hwcompare.com/11287/radeon-hd-6990-vs-radeon-hd-7970/
6990 only problem is that it's power hungry it's not a problem if you have a 800 W supply
but if you consider another one
^+1 An overclocked 7970 is running really close and surpassing the 6990/590 in some cases. You also don't have to deal with the issues of SLI/Crossfire.
There are very few situations where you'd need more gpu power than this, Short of a 5x1 or 120hz 3x1 or 3x1 2560x1600 setup.
For any single monitor setup it's going to be just fine. if you get 2x 7970 it will be a great choice