The performance is better but it wont be killer but as you said IQ is what you want.
Althoiugh some people will tell you the X1800XTs IQ is miles better than the 7800s there isn't too much truth in that but there certainly are differences.
Shader precision wise there is no difference at all. there used to be differences between Nv and ATI because ATI ran at a medium depth and Nv could run at High or low depths to provide better IQ where it is needed and faster speed when it is needed. For most things 16buit is plenty but in some situations it absoluterly will not do. Trouble is developers and drivers over used the 16bit mode. The 6800 ran 32bit all the time while the X800/X850 ran in 24bit so most of the time shader IQ was identical and very ocasionly ATI suffered from the lower bit deph. But nowadays there is no differnec e in shaders.
texturing wise, the AF texturing engine in the X1800 is superior to that of the 7800 as it is angle independent like the GF4/5800/5900 etc were. Nvidia copied ATI with their R9700 and started using angle dependent AF in the 6800 cards and up and this has lowered the texturing quality. With the X1800, ATI have gone back to the angle independent method. So ATI has better texturing quality.
With AA, this is very close between the two. ATI have long been regarded has offering better AA quality but this isn't true any more. Most reviews say Nvidia have better AA quality, you will also find many blind tests done where people in forums vote for the image they think is the best. I have seen this done 3-4 times on different forums, the Nvidia card always wins by a fair margin.
Compare these crossfire/SLI AA shots
ATI 14X
http://www.firingsquad.com/media/hi...e/ati_super_aa_vs_nvidia_sli_aa/images/11.jpg
To Nvidia 16x
http://www.firingsquad.com/media/hi...e/ati_super_aa_vs_nvidia_sli_aa/images/12.jpg
The ATI is very poor in comparison. Specially the Alpha AA. Nothing comes close to Nvidias 8X a and 16XAA because of the supe scene quality on NV hardware. Of course ATI's AA is faster in the x1800 so you may have situatiosn where the X18000 will allow 4XAA while your GT will only give 2X.