Let's be honest, as ATI's flagship product R520 didn't go too well. It wasn't that it was a bad product, it simply came too late to market due to an unexpected 6 month delay and on it's launch was priced much higher than Nvidia's 7800 GTX, of which had already been available at e-tailers for several months and therefore had plenty of time for its price to stabilize. By now everyone should know the delay to R520 was caused by a logic gate bug (circuit design bug) which in turn caused ATI's projected yield rate to drop significantly lower than they expected to achieve. Once the logic gate bug was found and corrected R520 gained 150MHz in clock frequency. Without this gain the X1800 XT would not have been anywhere near competitive to Nvidia's flagship of the time, the 7800 GTX. Of course, a few months after its launch prices settled and the R520 based X1800 XT became a very attractive card indeed. It is currently in direct competition with Nvidia brand spanking new 7900 GT and has become a superb performance bargain of sorts, but then so is the 7900 GT. Still, the story of R520 is one of bad luck.

While all these known and documented problems with R520 pursued, ATI's R&D were working on R580, the next generation 48 pixel shader replacement to R520, but the same circuit bug plagued R580 too. I assume the fix came in time not to affect R580's schedule, but very late in the day for R520, which is why the R520 and R580 launch dates were so close together (3 months apart). On the surface it all looks a little odd, but not once you understand what actually happened.

We'll be taking an in depth look at both R580 parts on the market, the X1900 XT and XTX. First let's take a look at what differentiates these models and then move on to the architecture.

Whereas with R520 the flagship model was the "XT", with R580 it is the second from top model, with the flagship having an extra X bolted on for good measure. Looking at the specs you should immediately see that the X1900 XT is identical to the XTX in every way except for the clocks speeds. The differences here are quite small, with the XTX only leading by 25 MHz on the core and 50 MHz on the memory. It's highly likely the XT will overclock to XTX speeds without much effort. The main difference between R520 and R580 are the amount of shader units present, which have jumped from 16 to a whopping 48. I will explain what this means on the following pages. Other changes in R580 include a few optimizations to the 512 bit Ring Bus memory controller and other minor tweaks

Price wise the Connect3D X1900 XTX comes in at around £370 with the XT version at £330, so the price reflects the fact that the differences between them are small. If you're looking at running CrossFire you'll need the CrossFire board, not surprisingly. It's clocked at XT speeds and comes in at around £375. Finally we have the X1800 XT, which is still a fantastic card and is currently priced at £250, though there are some better deals out there if you look around, plus there is a 256MB version available for even less. Like i've already mentioned, the X1800 XT's main competition comes from the 7900 GT, and brand new SKU from Nvidia, so don't completely disregard this card, it's still up there with the best of 'em.