If you're moving from IDE to 15K scsi, you're likely to see a pretty good performance boost even if you don't change anything.
I personally suspect there's not perfect answer. Do what your comfortable with.
FWIW, I'm running Raid 0 on two 15K scsi drives too. However, a good chunk of my traffic is script based and since I've got a good chunk of RAM everything is cached and the drives don't really work that hard. So I've got way overkill with the raid.
I've read people's concerns about raid mirroring as well, where you can still lose stuff even with raid. If cost is an issue, I'd say skip raid and put up with really good backups as scsi hd's are fairly reliable. If money's less of a concern, get the raid that makes you feel comfortable *and* keep really really good backups.
If you get raid, you run the risk of having two hd's go down. if instead you say use one hd for storage, and clone the drive to another say every 1/2 hour then you don't have the '100% uptime', but you're less likely to lose everything. If the first drive went down, you lose 1/2 an hour while you swap drives. IF your raid goes down, you lose from your last nightly backup and the time it takes you to source another drive and do a bare metal install. (actually, I run raid, and an onsite nightly backup, and an offsite nightly, and will probably be bumping my onsite backup to hourly - covering most of my bases).
Suggest you run some tests to see how much work your drive's are doing. I mean, if speed is what it's all about, and you're only using say 4-6 gigs of HD space, then slp some RAM into the server and run a ramdisk. Isn't that what Google does?
In terms of the CPU, my uninformed answer is that it's a tossup. And again, problably not going to make much difference unless you're under heavy CPU strain. Run some checks.
ALong the vein of 'nobody ever got fired for recommending MS', you're always better off to go high when buying your own servers. It's a heck of a lot easier to pay for more and faster hd's and faster cpu's now and grow into it, than it is to buy what you need now and outgrow it in two years. Then you'd have to swap servers- spend a couple of G's now and save yourself the trouble.(and if you're recommonding this to management, you should be suggesting top of the line. Then you can always claim 'i told you so' :) 0.