Street prices determine everything. No matter what the intentions, if 512MB 7900GT is clocked the same as 256MB versions and costs the consumer the same as a X1900XT, the 7900GT would be a lousy buy. You are right, X1800XT 512MB have been going up in price from the $260 they were a couple weeks ago. Quick search shows $316 at ewiz the best price shipped no rebates. But look at X1900XT coming down to about that price, which would mean any X1800XT or 7900GT near $300 takes a backseat.
The 256MB X1800XT is still out there for $235, up just $15. Buy.com had a 7900GT a few days ago for under $220 AR. If clock speeds are the same as this 512MB, then an extra $100 or so for double the memory is a lousy buy.
This card you link to is going to have to be either clocked higher than 256MB versions, or priced within $50 of the 256MB versions and $50 under X1900XT. Otherwise, IMO it's too little too late.
BTW, logic says don't buy a $500 Asus X1800XT, buy a different brand for less or an
Asus X1900XT for $350's.