Just checked, for single server use they recommend 4x cpu's minimum. As you've got HT cores available I'd throw a couple more on there.
It's also best to assign whole cores to servers instead of artificially limiting it in the virtualisation program.

An extra CPU Wouldn't be that expensive

You'll need to add some form of shared storage eg SAN for the failover too.

Can I ask again about backups

I'm not speccing any backups - it's not asked for in the brief, and it's their responsibility.

I've just been asked to spec up the hardware and a virtualisation platform. I've not even been asked/informed what software they are using to perform the E-mail/DB/Web server tasks, so they must be taking care of all that.

It's bonus points for spec'ing things like that.

Don't forget to spec an APC 3000 for the UPS & include references to software that may have been used in the reasoning (more points). A ESXi backup program is a must (saves a rebuild, then seperate restores).
Also include the carepack/4hr response, training, rails, labour (@ £700 a day, 5 days min), consultancy fees (@ £700 a day for designing it), the program you'll be using to virtualize the existing machines and and shortfalls of the plan (include a 2nd hotswap server for redundancy).

Probably best to also discuss why it's not better to virtualize 2 servers such as these to one server as it's actually cost more than replacing all 3 servers.

Thanks for all the help guys - I'm handing this in tomorrow.

Good luck.

BTW going back to the original question, I saw a demo of http://www.zenoss.com/ yesterday and it looks quite useful and would probably do what you wanted.