congrats on your new system...madcoo wrote:However, I'm surprised I don't get much more "Total Usable Device Memory" : both cards are approx 1Gb, so I should be getting double that amount, shouldn't I ?

you will have usable memory equal to the memory of the GPU, which has the least memory of all.. in your case, both cards are of 1GB, so usable memory= 1GB.
If you had one card of 2GB, other of 1GB; so max. usable memory= 1GB (least of the two..) similarly if you had two cards of 2GB each- so usable memory= 2GB
hope this helps...
it always helps to have 2 similar GPUs, having equal memory - higher the better