I will try and tackle both of these problems
First, the CPU. If you are looking to save some money and OC, then go to Newegg, buy the TBred 1700+ for $53 and get a nice HSF solution. I JUST built a nice system for an old friend that went like this:
ABIT NF7
Tbred 1700+
Corsair PC3000C2
Thermaright SK7
Sunon 80mm case fan
+extras
I was able to get that 1700+ chip to 2100MHZ @168FSB @1.8V and it was 47C underload (folding) and 100% stable.
OTHERWISE, get a Barton and dont even worry about OC.
Second RAID 5.
You have to have a minimum of 3 disks in order to run a RAID 5. You will loose the storage capacity of ONE of those drives but thtat is all.
You CAN move a RAID 5 volume to another machine AS LONG AS you move all the drives and the controller at once. You COULD get away with moving over 2 drives, but you would severly cripple your IO on that volume because then the CPU/controller would be responsable for rebuilding the data missing ON THE FLY.
Best thing to do is keep all the drives in the same config, move the drives and the controller to another machine and install the controller software into the new machine and the volume will still be intact.
From what you are prepossing in the rest of your question is possible IF and ONLY IF your controller supports adding drives to exsisting volumes and rebuilding ( I highly doubt that controller will though) I believe that feature is called FLEX RAID. My SCSI RAID adapter supports that function but then again it retails at around $450.
I would suggest you copy all your data to another source, (network computer, another HDD, etc) setup your 100GB + (2) 120GB drives in the RAID 5, then move the data onto the new RAID 5 volume. NOTE: you will lose 40GB worth of storage space on the 120GB because the RAID card will only allow you to make a 300GB RAID 5 volume due to having a 100GB HDD in the array. You also might have some compatability problems between the drives, but that should not occur.
Alright I hope that that helps you out. Good luck.