It depends on what file system you are using. I havent quite optimized my system for NTFS yet, so if you are going to use NTFS, then I would ask somebody else. On the other hand, if you are going to use FAT32, then I would suggest a 16KB stripe size and 16KB cluster size for your file allocations.
I just got done reading Adrian Rojack's RAID review and 64K seem to be the worst. It looks like 16k is probably about the best compromise. I haven't benched my current raid at 16k yet but when it was 64k the drive index came out at 20k for the onboard Highpoint Raid - which is way below my original Raid using a PCI promise card, that gave me a DI of 31k so I am quite disapointed in my new Soyo P4S board's RAID performance. I tried to use my old Promise PCI raid but I can't get the Soyo to load it's bios