The high-end X1800 XT will be clocked at 600MHz with sixteen pipelines and memory clocked at 1400MHz. These cards will barely be available as ATI has problems to produce enough quantity of these.
The X1800 PRO will be clocked at 550MHz with sixteen pipelines and memory clocked at 1250MHz. This card should be widely available.
Then there's also the X1800 XL which will be clocked at 500MHz and 1000MHz memory. This card with also sixteen pipelines will be slightly cheaper and should be highly available.
It's possible ATI will launch even more R520 based cards.