I have two X1800 Crossfire systems running now. Both on a 480 Watt PSU. Both are happy.

With 7800 512MB the same PSU would not power them in SLI.

I tend to agree with the feeling that X1800 XT is better for gaming. I cannot figure out why but IQ seems better on the ATI cards. Most screen shots show them the same but in game I can see the difference. Maybe it is in my head.

I am curious about X1800XL CF as well. I have not received confirmation that there will be a specific Master card for them. The X1800 XT master card will work with them but it is a question of cost. (One cool thing is that you could buy the X1800 512MB Master card...run it with the X1800 XL and then upgrade to the X1800 XT 512MB card later.)