@sumul, the problem with that is that you’d need to buy a copy of windows to install on Parallels or VMware Fusion (unless of course you’ve got a spare copy of windows).
I’ve been looking into this myself recently and in the end, the best way I found to run IE6 & 7 on OS X is to use the VPC images supplied for free by Microsoft to test IE6 & 7. The only problem is that you’ll need to set it all up again when Microsoft expires and then re-releases the VPC images (currently set for Sept 08), but it’s a small trade to be able to set it up for free…
There’s a guide for how to set up the Microsoft VPC images with VirtualBox of VMware Fusion here (read through the comments to find how to do it all on OS X using Q.app). Completely free if you do it with VirtualBox, $40 if you go for VMware Fusion.
I installed MultipleIE on one of the VPC images and when comparing it’s version of IE6 with the original from the VPC image, it actually does render quite differently (esp. lists). I used to trust MultipleIE at my old work but now I’m not so sure.