For a brand new one, sounds OK. Would need the whole set of specs. You could think about one more drive or a hot spare. 2008 and Windows will take up at least 80GB or more of the 500GBs you have now.
RAM is cheap. SQL loves RAM. Since the big upside to 64-bit is having the ability to go to 64 RAM instead of 4, I would consider going to 8 or 10. I have 20GB -- which is definitely overkill, but at least 8 would be nice.
But, why not SBS 2008. To me, that's a no brainer. To have Exchange Server 2007, SQL Server 2005 (not Express), Sharepoint and RWW all for free is unbeatable. I would definitely, definitely, definitely go with Small Business Server.
