Hi Andrew, it looks like only the full version of ESX 3.5 is supported on the x3400 model with the Intel 5300 series processor. FYI, I used the HCL at the address below (page 20). It was updated just a few days ago, so it should be fairly accurate.
http://www.vmware.com/pdf/vi35_systems_guide.pdf
Hope that helps! Please help me out by marking my response as "helpful" or "correct" if you feel that it was useful!
-Amit
Please help me out by awarding points for a "helpful" or "correct" response if you feel that it was useful!
-Amit