At the same time as their Radeon X800 XT launch, ATI also launched a new mid-range part to satiate the needs of the masses. Based on the same R420 GPU that powers the X800 XT, the X800 PRO employs the scalable characteristics of R420 in its execution, implementing 12 pixel pipelines (3 quad pipes) to the XT's 16.
R420 GPUs are tested for defects, with completely functional parts sold as XT and the less able parts sold as PRO with one quad pipe disabled. Similar scaling will eventually produce 8 and possibly 4-pipe R420 products, when the market dictates they should appear and yeilds are high enough to support them.
At $399 (£300 in the UK, you can preorder PRO boards at the time of writing), it's more high-end than mid-range in terms of price, but it's a measurable jump from PRO to XT in terms of performance.
Just how much of a jump is what's important, given current early-adopter pricing.
If you've followed the R420 launch on HEXUS and other websites, you'll be somewhat familiar with the architecture and how it goes about its business in hyper-R3x0 fashion, taking the existing performance of their Radeon 9800 XT and doubling it (for the most part), while adding a smattering of new features.
The focus of this article is on performance rather than image quality or architecture, to save me repeating myself and you reading information that you've already digested.
Check out the original article on the X800 XT if you're interested in how it works.