IO does not have anything to do with dynamic (or any other) RT in FCP 4.x or 5. The only thing IO does is what it’s name says I(nput)O(utput) and nothing else.
If you need more RT power buy more memory (but it will not help you as you already have 4GB), faster computer (well, not much help if you buy a 2×2.7GHz) and faster disks (to be very honest XRaid is not my favourite, dual channel SCSI U320 is!).
Hope this helps you,
Jadranko