I found this while looking for another article about hacking into OS-X
http://blogs.zdnet.com/security/?p=758 it's a little out of date, but certainly does highlight that OS-X is just as vulnerable as XP/Vista. The most secure O/S is usually built on a linux kernal. The article I was looking for came from a hacking show where fully patched O/S went against each other. The hackers cracked OS-X first, followed by Vista and then Ubuntu.
Although by no means am I a supporter of Microsoft or it's operating systems.
Paul, it does depend on your budget, I would recommend a Dell Precision work station but they start at around £700.