I'm also on W7 x64 ultimate. Loaded up Thunderbird and it works OK for me. I ran Office 2007 for about 10 days then got Office 2010 through a deal at work, so loaded it up. OL 2010 seems like it has a learning curve, but I'll be OK.
I've always used whatever mail client I like set up as IMAP, and then just archived with Outlook. Seems like a backwards process now, the more I think about it.
