SATA stands for Serial ATA. It's the new drive interface that replaces EIDE which most computers have today. It's a much faster connection and supports things like command queing which allow the drive to essentially cache read/write commands.
So far, most of the SATA drives didin't really take advantage of the SATA specification - they were basically EIDE drives with the new interface added to them. This drive doesn't even have the older EIDE interface or power connector...
I ran a benchmark to get a feel for the performance and it seemed to be roughly 20% faster on my machine. I can't vouch for the benchmark software though.
Of course a fast drive isn't really needed for playing music files, but I wanted to try SATA (my motherboard supports it natively) and it was the only drive I saw that was 300GB. Plus, it was on sale!
My HTPC case is still just sitting there, but each new item I buy (like this drive) will find its way into it once I get a motherboard for it. I've had a preamplifier, amplifier and speakers that I am happy with for quite a while now. So it's basically just waiting for a motherboard and power supply. I eventually want to build a RAID 5 array for all of my media, but that's going to be a while due to cost.
Question for you: Are you planning on using your HTPC for recording TV shows as well as listening to music? If so, it might be worthwhile just to build the HTPC now and hook it up to your existing system, then replace speakers/amps/etc. later.