Yes, I agree with Thilips. The later distributions should support SATA better. The GX620 should also more then powerful enough for you to try the new Compiz or Beryl 3D desktops with the FC6 distribution.
Any particular reason for sticking to FC3? If not double that number
EDIT: Ooops, I presume you have upgraded several times by now Bagatur? Didn't read the date on the original post at first... :-)