I wouldn't think you'd see much difference in speed to tell the truth, no. Its just another way of transferring data, though it is suppsoed to have faster speeds. I guess it is like AGP and PCI-E at the moment, there is no hardware that will push it that extra bit farther, though PCI-E is supposed to be better
You will notice a difference a small bit in windows load times and installation times, and uninstall times, but nothing heavy. HOWEVER, if you get an SATAII drive, you WILL NOTICE a significant difference. nForce4 Ultra and SLi Deluxe supports that, which in fact are also PCI express. No, if you switch to SCSI HDDs, you WILL NOTICE A HUGE DIFFERENCE, but you better have a LOT of money.
I agree. If you have motherboards that support SATAII such as the Abit FATAL1TY AN8, you will see a boost with SAmsung's new SATAII hard drives coming out. THeir new drive is fast because of the increased I/O speeds that goes up to 3.0Gb per second maximum.