I don't yet. But, I will be buying me a copy of Vista Business sometime this year.
Why?
Because it's the latest version. I know, it sounds like a bad reason. Regardless, it will become a defacto requirement. When XP came out in 2001, it started out as optional, but now it's effectively required. Vista may not be necessary today, but it will be.
Call_Me_Daddy: Buy more RAM. If your computer is like most, it probably uses the cheap Non-ECC Unbuffered stuff, so you don't have a lot of excuse.
The problem you mention isn't new. It's been that way for years. In the mid-90s, a PC with 64MB RAM would've been good enough. Not anymore. Software only gets more and more bloated as time goes on. WordPerfect 5.1 could do anything I need a word processor to do, and then some. WordPerfect 12 isn't vastly different, yet it takes up a lot more space and needs a beefier PC.
I know what I say sounds crappy, but it's true and it affects all of us. In 1998, I assembled a PC at a cost of $1,500. Now, it's barely usable for certain purposes, and unsuitable for daily use despite it now having a doubly fast CPU and four times the RAM it originally came with. My current PC, assembled at a cost of $1,750 in 2004, probably only has a year or so of life until it's unbearably lacking, short of some serious upgrades.