given that you're using a Mac, i assume u run VMWare or Parallels to test in Windows? Create another VM with the alternate browser in it.
So you'd have one with Vista and IE7, one with XP and IE6. you MAY want to also test in IE7 under XP (yes, another VM) due to potential differences in the versions. (i have yet to test this myself)
microsoft's solution is to run a VPC image of XP with IE6 on it. thats kinda useless if you're already running Vista in a VM on your mac.