We run our SQL servers on VMWare and have been very happy with the performance. BUT, we are a small organization with a lightly loaded primary SQL server with a few hundred users, most of whom are either reading small amounts of data or updating selected rows.

We subcontracted most of the VMWare setup to a consultant who is very experienced in setting up SANs w/ VMWare servers, which has been essential for our success.

If your primary app. is so poorly written, is it a resource hog? Something I didn't see you mention: if you virtualize and SQL server is sharing CPU/RAM/IO with other servers, then the other servers may suffer if you aren't careful.

I like the previous suggestion to create a parallel test environment. I know you said upgrading your application is not a possibility, but it honestly sounds like you'd likely get more performance bang for your buck by doing that than you will by virtualizing. Maybe if you ran the numbers, you and management would have something meaningful to compare?

Good luck,

Rich