RAID -= no real value in my (uneducated) opinion.
Powersupply 450+ so that when you decide to upgrade you will have a good basis.
extra £30 or so for a faster CPU? Depends on if you will be cutting corners to afford it - an extra 256 of RAM would be better; both together best of all.
Don't buy a dell if you want to upgrade it yourself... build your own because even though you will not save money, you will know get what you want in every way.