Get the xt not the xt-x, don't bother with the crossfire solution later as you can upgrade to a gtx for the same performance. The price point will be 40% less with the ati xt over a nvidia gtx but clocked will be only 25% slower.
Sell the card later and get a gtx and you will have won on all points when the prices of the gtx reduce.
As for the psu, spend a little more and get yourself something that will last for years to come. The OCZ 700w game extreme is an excellent choice, indestructible and quality parts.
Works out at more or less the same price an extra tenner gets you the Hitachi deskstar 320 gig, so an extra 70 gig for a tenner, not bad.