DailyTech has a story up about the upcoming six core Dunnington Xeon CPU, which is a replacement for the fairly recent Tigerton Xeon CPU. This new chip is pin and chipset compatible with the Tigerton Xeon, so it should work just fine with a Dell PowerEdge R900 server when it is released during the second half of 2008.
The CPU should absolutely smoke with an OLTP workload with SQL Server 2005 or 2008. SQL Server loves large L2 and L3 caches, and it runs very well on the 45nm Penryn based architecture. We have been doing quite a bit of testing with the February CTP of Katmai on a single, quad-core Xeon 5450, with very good results.
CPU-Z Shot of 2.83GHz Xeon 5450 "Harpertown"