I'm running Parallels v6 (so v7 would be even better) and it's pretty solid. 2.3Ghz i7/quad, 8GB RAM, 7.2K HDD with my VM setup for 4GB.
I'm using Coherence mode mostly, where it puts the Winders© windows on the Mac desktop, even has a theme it can deploy that really blends them in aesthetically. All my Win7 system/notification icons are right up in the Mac menu bar, plus I have access to my start menu right from the top menu, or a folder on the dock.
I'm running VS2010 and a couple of DB tools (SQL Studio and EMS for Oracle), plus an instance of IIS and SQL or Oracle), along with all my native OSX apps I keep open like Mail, iCal, iTunes, and Chrome.
Easy resource sharing, and in full screen mode, I can easily access my "core" OSX apps from right within the Windows native start menu.