So I've been thinking about going to Vista Ultimate 64bit for a while now, but after talking to my uncle who works at Microsoft I dowloaded the Windows 7 RC (Release Candidate) 64-bit.
Windows 7 RC Download Link
So far, I'm liking it! I just put together a new computer with 4 gigs of ram and an Athlon 64 X2 processor, and have been itching to take full advantage of the ram especially (I'll be upgrading to 8GB soon, along with a GeForce GTX 260). Overall, it's a good upgrade from trusty Windows XP Pro, although searching for drivers for the 64-bit system may prove difficult for some of my legacy hardware.
The RC is good until next summer, at which point you either have to go back or buy the released Windows 7 at that time. Definitely worth it IMO, as long as you don't mind making a dual-boot partition or formatting you hard drive (like I did).