The x1xxx series of cards are the new ATI shift of manufacturing processes. These are the new flagship cards for ATI and are pretty much for those of us who can't stand having anything less than perfection and bleeding edge technology. It's worth mentioning that monarchcomputers.com has listed this:
ATI (HIS) Radeon X1800 XT 256MB GDDR3/PCI-E/Dual-DVI (Retail Box) for $369.69
That's a screaming deal. This is because it's not the 512 MB of RAM that the other models this high up in the food chain are offering...which is granting them a ton of publicity. It does, however offer the new technology called "Shader Model 3.0." This is the newest in gaming and is something every new card from now on will have. (The X800's don't have this). This brings increases in performance.
As for a power supply, you should be fine with at least 350W but it's always nice to have that buffer zone just to be safe. I use 400W and would use that on this card with no problems.
Okay...So you also want to know about the X1xxx series from ATI and them compared to the NVidia 6800gs. First off you need to know this:
Frames per second Gameplay
<30 FPS very limited gameplay
30-40 FPS average yet playable
40-60 FPS good gameplay
>60 FPS best possible gameplay
(Stats taken from guru3d.com misc. reviews)
You also need to see these graphs of average FPS they got from Far Cry on a comparison basis.
Based on this, I would say this:
If you plan on getting a new graphics card, spring the extra cash for the X1800 XL or XT. Obviously, the 512 MB version would be preferred over the 256 MB one, but that's up to you in the end. These cards kick some serious you-know-what and you will definately feel like you got some really cool stuff for your money. The 256 MB x1800 I listed above will be better than the x850, and relatively the same price.
As far as buying a pre-built system goes, I like HP. My tag below my posts shows what I use and I'm happy with it, minus the need for a faster processor and one of those new x1800 cards on of these days. The HP's will most likely come with a 350W power supply, so there won't be a need to upgrade.
Phew, if I missed anything or you need some clarification, let me know...ha