Your OS would be better on the velociraptors in a raid 0 then it would be on a single 500gb hdd.
They do make 80gb VelociRaptors. You could stripe two of those and get 160gb. They run about $60 on amazon.com now, and in a raid 0, would net some descent read/write speeds. A pair of 80 VR's should read in the neighborhood of 200MB/sec and write probably 150MB/sec. But, if you're going to spend $200, you're almost into SSD territory now. You can get a 128gb SSD that would blow away a pair of striped VR's for like $220. a 160gb SSD would prob be around $260 starting out.
Food for thought lol, i will have to see what my tax return is then decied what to do.