Meanwhile,
I have purchased some second hand 7950s from ebay which are much cheaper, run colder and almost as fast as the 7990s. The biggest problem i have had with the 7990s is the massive heat buildupm. Keeping them under 80c is a challenge, so I have had to underclock and undervolt the 7990s to best extend the life of the cards.
Im thinking watercooling is essential to get the most out of a 7990.