The joys of the surgical (and most vettable, unfortunately 
approach of collecting empirical data, analyzing vendor recommendations, and making the best selection. It's a painful, painful process when making this kind of move - and very scary, considering the staggering amounts of money involved, especially if you're on a shoestring budget. ESX4, through storage vmotion and thin provisioning can provide you with a lot of flexibility and functionality margins for maintenance, but neither of these can necessarily provide you with great performance - at the end of the day, spindles and storage handling mechanisms will dictate that. And that's what these numbers are all about - what kind of real 'running load' performance is happening with your machines today. The vendors, once you feed them these numbers, will most likely feed them to their sales engineers / technical engineers, and from there a recommendation will be made for a unit that matches your shop's load.
Great article explaining how ESX4 and all the major aspects of storage performance ties together can be found here:
Remember, storage metrics are calculated in IOPS. Also remember, everything I've babbled on about is pretty much just a primer down at the physical disk level - you're also going to need to consider if you're planning on an iSCSI, FC, or NFS-based deployment, or a mixed environment. That opens it's own cans of worms...
Unfortunately, I think as you explore this question, you'll find the real answer is 'it depends'. I realize this isn't very helpful - but you're right on your first steps - collect the numbers and start doing your homework.
My two. Good luck trooper.
- abe
Integritas!
Abe Lister
Just some guy that loves to virtualize
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