There is no driver available to do a fresh install with the beta 64-bit Opteron versions of Windows. I beleive Intel had their hand in this. However the driver is in the beta after loading the OS on another drive. I took the latest XP/2003 driver disk from LSI's website and replaced it with the 64-bit one in the Beta (MRAID35x.sys). I was then able to boot in recovery mode with this floppy.
Extract the included files to a floppy and press [F6] on your install to load 'em.
Should support the following LSI controllers under Windows XP 64 and Windows 2003 64.
LSI Logic MegaRAID Express 300 RAID Controller
LSI Logic MegaRAID Express 500/500LC RAID Controller