No, it won't. The interface still isn't the bottleneck when it comes to HD speeds. A harddisk that does 40-50mb/s can be considered a VERY speedy harddisk. I don't even think there are harddisks around that are faster than that (yet). Considering the IDE interface does 133mb/s max, you've still got a lot of margin left. This margin shrinks with two drives on the same cable, but it's still adequate. Only in some IDE RAID setups, the ATA133 interface is not speedy enough.

Also, you have to know that almost ALL serial ATA harddisks that are out now (with the exception of Seagate S-ATA HD's) are in fact ATA133 harddisks that use a serial bridge chip to be able to use the S-ATA interface. So serial ATA disks CAN'T be faster than their ATA133 counterparts, because they are the same disks.

So what are the benefits now for using S-ATA ? Thinner cables... Yup, that's about it.

In the future (more specifically with the S-ATA II standard), other clever stuff (that now already exists with the more expensive SCSI drives) will be implemented in the protocol and then there WILL be a speed advantage. But not just yet.

This said, I would like to point out that I use S-ATA disks myself, and the thinner cables alone makes them worth their price IMO.