promise ultra 66 to fastrack raid controller; that's a well known conversion that is easily accomplished w/ a BIOS flash, a small soldering iron and some conductive tape...
throughput on hpt 370 raid controller is roughly 40% faster than a promise ultra 66 ide controller.
Read Tom's article at <A HREF="http://www6.tomshardware.com/storage/00q1/000329/index.html" target="_new">http://www6.tomshardware.com/storage/00q1/000329/index.html</A> before you make a decision. He suggests that it will provide only a minimal performance increase.