I thought this might interest some, particularly after all the debate some months back where we were all told how much better the X1800XT was by all and sundry (I seem to remember hexus gushing over it..). I've recently swapped out my 7800 GTX 256 for a x1800xt and here's my initial thoughts on differences:
1. Speed
Exactly the same as far as I can tell. Everything I've played so far seems no worse or better than the old card. If you're think of upgrading for this alone, don't bother. My marginally overclocked GTX (510 core) felt a lot smoother in HL2 lost coast for example. I've not overclocked the XT yet, simply because i've only just fitted a watercooler to it and not had the chance. I'm sure overlocking will even this out..
2. Quality
Image quality is better - you might not gain FPS from this card but you can turn on more levels of antialising/AF without suffering.
3. Temps
It's a hotter card - make no mistake. Core temps at idle are much higher and i'm sure you can fry food on it when it's under load
4. Physical attributes
It's HUGE. The 7800 GTX looks incy-wincy compared to this double-slot monster card. It weighs a lot more but this is down to it's huge cooler - which by the way is neat in that it ejects it's hot air out he back of the case directly.
5. Noise.
Often overlooked this - but this is one noisy barsteward. The first thing you notice when powering up your PC is that, apparently, you've fitted a dyson to it. Whhhhhhhhhiiiineee..
GTX was much quieter again here. Watercooling became and essential here (but at least the fan does dynamically ramp up/down so you can hear people on the phone [when not playing games]).
6. Drivers
I've never had problems with ATI drivers since the Rage 128 days and no problems here so far. I _hate_ the .NET control panel (catalyst control center) as it's fat, lardy and slow (.NET for you) but then i'm a programmer and such things bother me.
7. Warranty
nVidia cards come with 3 years, ATI with 1. Not good ATI!!
Watercooling:
I went for an Alphacool unit:
It's a Alphacool NexXxoS NVXP-3 6800/7800 but it's often advertised as fitting the X1800XT too. Whilst this is true, it's not perfect as you can only use the four core screws to clamp the unit onto the card (the 5 surrouning outer screws do no match holes on the board) and this can leave some of the RAM chips without proper contact with the base of the cooler. I solved this by adding some Artic Silver 5 to the themal pads (which you place on the ram before hand) to 'bridge' the small gap. Not ideal but better than a mm gap on the odd chip (and hopefully this works). Core temps at idle are now 31/32C which is a drop of ~32C over the stock cooler.
So why did i do it? What did i gain? Probably only double the video RAM, other than that nothing of note. What did it cost me to do it? Nothing, I got XT for ~280quid, which is what i sold my card for.
Which one would I go for if buying now? The ATI one. The 512 GTX is just silly money (as is the 1900XT).