Is using the /3GB statup switch in Windows 2008 x64 only running Sql Server 2008 x64, a good thing?

The /3GB switch does not apply to 64-bit editions of Windows

http://support.microsoft.com/kb/294418

Applications that are compiled with the /LARGEADDRESSAWARE option, as would be required to take advantage of the /3GB switch in 32-bit Windows, will automatically be able to address 4 GB of virtual memory without any boot time switches or changes to x64 Windows. Plus, of course, the operating system does not have to share that 4 GB of space. Therefore, it is not constrained at all

and

A pool of system Page Table Entries (PTEs) that is used to map system pages such as I/O space, Kernel stacks, and memory descriptor lists. 64-bit programs use a 16-terabyte tuning model (8 terabytes User and 8 terabytes Kernel). 32-bit programs still use the 4-GB tuning model (2 GB User and 2 GB Kernel). This means that 32-bit processes that run on 64-bit versions of Windows run in a 4-GB tuning model (2 GB User and 2GB Kernel). 64-bit versions of Windows do not support the use of the /3GB switch in the boot options. Theoretically, a 64-bit pointer could address up to 16 exabytes. 64-bit versions of Windows have currently implemented up to 16 terabytes of address space

With SQL Server the more memory the better. SQL Server will pretty much take what ever you can give it. You'll want to reserve at least 1 Gig for the OS, and leave the rest for SQL Server.

Because its the 64bit OS you won't need the 3GB or AWE switches for the OS.