Hmmm... ohhh-kayy... have you tried adding the megaraid-mbox driver as well?
There is a suggestion that you may need updated firmware for the perc - but your perc5 should be fine.
There is always installing without RAID (switch RAID off in BIOS). Then, if you want RAID, you use linux software RAID which is usually faster anyway. As this is sata raid, try setting bios to legacy mode.
Other distros may be easier:
Naturally you can get RHEL4 working - dell supports this distro. This suggests that CentOS should work as well. Have you tried Ubuntu?