Dell :: Inpiron 1520 SATA HDD

Recently, I ordered Dell Inspiron 1520 with the following main specifications:

Intel Core 2 Dou T7500 2.2-GHz CPU
2x1 GB 667-MHz RAM
nVidia 8600GT 256-MB GDDR2 GPU
Fujitsu 160-MB SATA 5400-RPM HDD
Microsoft Windows Vista Home Premuim OS

Anyway, at the beginning I was really disappointed with the performance; the system was too slow specially during booting, loading, and playing games. Furthermore, when the notebook is on battery, the performance was terrible; I even can't watch movies smoothly or explore the hard drive normally.

So, I entered the BIOS, load the defaults, no improvements!

Then, after too many calls/chats with Dell technical support, someone asked me to update the BIOS, then I update it, but again there were no improvements.

Finally, I got the right man from Dell support, and he suggested to change a BIOS setting, which is the "SATA Operation" and set it to "ATA" rather than "ACHI". Then, I entered the BIOS (which is the new version "A03"), and went to the "SATA Operation" option then I saw the default factory is set to "ATA" where I am sure that it was "ACHI" as a default factory with the previous BIOS version. Anyway, I set it to "ATA" and I had to disable also the "Flash Cache Module" to do so. Then finally, the system performance improved and everything seems to be normal.

Now, my questions are:
Did I lost the "SATA" feature/performance of my hard drive (because I set it to "ATA")?
If I do lost the "SATA" feature/performance, is there any workaround solution to make the "SATA Operation" as "ACHI" and system remails OK (e.g.: installing new storage/interface drivers to the OS)?
Why in the new BIOS, the "SATA Operation" default factory is set to "ATA" not as like the previous version were the default is "ACHI"?
Is there anybody faced the same problem as mine? If so, how did he solve it?
Is this one of Dell disadvantages/faults?