Just wondering if anyone has upgraded from a regular hard drive to a SATA,and
if you've noticed improvements.

Yes. I've recently upgraded from an IDE drive to an SATA drive, and there is a significant improvement -- if you know what to look for. I doubt most home users will notice anything drastic, but the speed of the drive under heavy disk I/O is definitely improved.

I haven't upgraded myself, but I now someone who has two SATA drives. While I don't know if it is any faster than it would have been without SATA (it's pretty fast anyway, as he uses them in a RAID0 configuration), I do know it's a hell of a lot easier to work with SATA cables than it is to work with the old IDE flat cables.

I have Serial ATA hdd - Seagate Barracuda 7200.7 160GB. But, to be honest, there is only "cosmetical" difference in speed between this drive and other Barracuda on old IDE. But there are other very nice features of SATA drives:
1.Thin connection cable (better air circulation in the case)
2.Hot Plug function
3.No jumpers
4.Every device works on its own channel, so there is no problem like with IDE devices, when you connect slow cdrom drive with fast hdd
5.You can be sure you will be able to connect your serial ata hdd to your new mainboard in the future

You will not notice any "real" difference if you are looking at speed, the transfer rates are mainly capped by how fast the drives can send/recieve the information. If you are looking for speed improvements then you can boost performance by getting a faster hard drive, check storagereview for info. I have a WD Raptor which is a sata drive and currently the fastest in the IDE catagory.

SATA offers benifits that are probably not evident to the average user, ie - no master/slave settings, easier cabling, longer cabling, hot-swapping, etc. It will become the new standard but you wouldn't see a difference in terms of actual performance.

is this drive loud ?
i want to buy the 200gig version of that native sata harddisk

Barracudas series are known as a quiet drives and the same is with mine hdd.. Even on full load it is very quiet. But it also may depend from your case- I have extra stable Chieftec case [17kg (!) weight] so there are nearly no vibrations.
And this hdd has 3 years warranty period (at least in my country).
I think about buying second the same drive and connect it to the Raid 0 . My system is high loaded very often so I may have benefits from it.