Most general consumer hard drives still have transfer rates that are <150MB/s IIRC, so SATA150 vs SATA300 would make no real difference. It's been a while (okay, like 2006) since I was fully immersed in the technicals of HDDs, but a drive such as (
www.storagereview.com/western_digital_caviar_black_review_2tb) has a listed transfer speed of 138MB/s which falls below that 150 threshold. Transfer bursts may come closed to exceeding that, and 10k rpm hdds as well as SSDs are likely to exceed SATA150 capacity, but most general drives do not, and therefore whether theyre sata150/300 are irrelevant. Try to find actual technical details on the hdd you're looking at purchasing - sequential reads/writes, random reads/writes, cache size, rpm, and latency are largely going to determine overall performance. Someone feel free to correct me if my technical understanding has lapsed in my old age. :P
On another note, I just put one of these in my Macbook Pro @ work, and damn is it bitchin':
www.newegg.com/Product/Product.aspx?Item...20-167-024-_-Product