I had the same problem with u with my KT7A-Raid, I was getting like 13000 in Sisandra and I noticed the HPT370 driver is the default one for XP, so I updated the driver to Highpoint's 1.0.3b driver and I got a boost back up to 34000. I'm not sure if it's right to do to use the driver other than the one provided in xp but so far I've no problems, but I've only been using it for couple days.
PS. I wasn't be able to get it to 40000 in Win2k either, weird, people are getting over 40000, and I did nearly everything like disabling ACPI, stripe size set to 8, etc, but no luck. Oh well... seems like I'll have to live with it, at least it's better than 20000 when I first got it.