What Alienware PC is it? An Area 51? If so I think that comes with a H50 anyway, or a system thats basically the same so not much point going for that if your looking at adding graphics cards in to it as well. But as neo-omega said, make sure your board can support SLI as well as crossfire (think it should but best to make sure), wouldnt be fun buying those 2 295's to find out they wont both work in there!!
As he said as well, dual loops may be a good idea. Will cost a bit more and be a little more complicated, but offer much better cooling performance. Those 295's will be putting out a lot of heat so maybe best to keep them in they're own loop for better cooling. Would also mean you could get your CPU/NB loop set up now, and then just add another loop for the cards when you get them, rather than modifying the current loop.
EDIT: is it this case?