Well looking at the current RTPE pricing, the cheapest X1900XT is $480 right now, not even with any rebates.
The cheapest 7800GTX is $435 and that's 256MB. The cheapest 512MB 7800GTX is $690 and it's a pre-order aka non-existent.
The 7900GT coming in at $400, while expensive, is par for a 7800GT replacement. The 7900GTX though, unless it outperforms the X1900XT, can't be more than $500 without being overpriced from the get-go, especially considering the X1900XT prices will continue to drop, especially with a 512MB competitor (7900GTX) actually available in stores (assuming NVidia can come through with supply quantity this time).
So if you release a new product at $500 and your competitor's product is already at $480, unless you're outperforming them or offering some additional features, you're going to have to drop the price right away. The price differential is even more exaggerated if it comes out at $550.
Now consider if they do bring it out at $500. That's only $100 more than the 256MB GT model. That should do one thing: force the pricing down on the GT model to $350 so there is a greater price differential between the two. Since they can't go UP in price with the GTX, they'll likely have to come down in price on the GT to maintain the $150 price gap we saw with the 7800GT and GTX (and those were even both 256MB models).
Should be interesting to see what they end up doing for MSRPs on the GT and GTX.
I'm still expecting $400 and $550 and performance on the GTX on part with the XT in most games, better in Q4 and B&W2, worse in FEAR and Source engine (HL2). Then the GT will come down to $350 at the end of March when the X1900XL comes out, probably also at $400, and offers very strong competition for the 7900GT.
That's when I'd buy a 7900 (especially the GT) - at the end of March after the X1900XL has been out for a week and reviews are up showing how it compares to the GT. If the XL wins, the GT's price should start to drop a bit and would be a great time to grab one. On the other hand if you're an ATI fan, you already have an XT (or are waiting for a more quietly cooled XT lol).