I'll wait till tests before saying the X1800 and the 7800 GTX are equals...
The X1900 mobile is supposed to one up the 7800GTX and thought I read somewhere (sorry I know I should have a link but can't be smart enough to save it all) that the X1800 was supposed to come in under the 7800GTX in performance (probably more like the 7800 Go vanilla) but with better power useage than the 7800 GTX.
Tests, though are the real deal and we should be able to test soon it looks like.
I dont know if this is any indication of price. But i was poking around a German site that had a duo core with 7800gtx and i configured it with a 2.0ghz, 2gig ram , and a 100gig 7200rpm HD and it was about the same price as the single core after doing the Euro to USD exchange rate