unless the thread topic is about buying hardware in australia (or wherever) it is quite useless... in this case, the OP is not in Aus. so those prices are irrrelevant.
for the GT to be equal to the XT at stock clocks, the GT needs to run about 550mhz core, and then the GT still lacks in texture filtering and ability to do concurrent hdr & aa (and some will further claim texture shimmering is more of an issue with the GT than the XT).
the XT does use a bit more power and certainly expels more heat (tho it vents it out of the pc case). as far as noise, the GT does not have the nice fan the GTX does, and while the actual decibels are similar, many people claim the XT is more noticeable as the "pitch" of the sound coming from the fan is more notcieable when the fan changes speeds.
from my own perspective, i've gotten used to it, and while every once in awhile i do notice the fan change speeds, it's not really bothersome (my pc is otherwise liquid cooled so it's very quiet in the first place, and sits on my desk about 18" away from me at ear level).
aside from those things, you'll simply have to do some research on current street prices to determine what you feel will be your best "bang for the buck".
for those that suggest the 1900GT, it is not only slower, but currently more expensive than the x1800xt 512 (at least here in the us). since the GT is going to replace the 1800xt at that pricepoint, if you decide to go ati, grab the XT while you still can.
and why is it there's a new thread on this very question almost daily? this topic has already been discussed to death in multiple threads...