What speed does your harddrives run at? If its on the lower end this could account for longer load times regardless of them being in a raid 0 setup.
Benchmarked the drive volume with a read performance of over 150MB/sec, so I find it hard to blame the drives themselves.
Actually, the problems I have seen with this are often directly related to drivers "misreading" the greater than 4GB RAM and reducing performance considerably, despite the use of a 64-bit OS. A prime example of this occurs with Vampire: Bloodlines where on systems with newer Nvidia hardware/drivers and more than 4GB of RAM, the drivers automatically reduce texture quality and performance to extremely poor levels. Setting the MAXMEM flag corrects that situation, so if you are getting something similar, it could resolve your issue.
Like I said, its a bizarre suggestion, but at this point, the only other thing I think we have left to "blame" is Vista 64, which doesn't really solve your problem.
One never ceases to learn. Quite interesting facts you got there. Makes me a bit more optimistic about the MAXMEM thing. I'll give it a try and report back as soon as i have the time to do so.