For a PSU I recommend the Seasonic S12 series it is really quiet. If your going for an SLI setup in the future then get the S12 500 which is SLI certified. If not then you can go for either the S12 330, 380, or 430.
Seasonic S12 330 has more than enough power to run an Athlon 64 X2 4800+, 7900GTX and 3 hard drives, DVD-Burner, soundcard, motherboard, 4 sticks of RAM. If you were going to build a PC around Pentium 4 then I would recommend the S12 430 since P4s consumes at least 70% more power than a comparable Athlon 64. Prices starts at about $60 for the S12 330, the S12 500 is around $130, and the S12 600 is around $170.
For fans, arguably the best quiet fan to get is the
Nexus 120mm for $16 + shipping. Other options includes Yate Loon, Papst, and Scythe all 120mm. Those clear plastic LED lit fan are generally louder than traditional solid color fans because they use hard plastic that do not flex. That can cause air turbulance which will be audible. Regardless what fan you get, you should also by the Zalman Fanmate 2 to control the voltage for reduced RPMs.
As for the case, many people recommend the the Antec P150 because it allows you to suspend up to 2 hard drive (IIRC). Suspending the hard drives reduces vibration noise because the hard drives are suspended by "rubberbands" that absorbs the vibrations. The P150 also comes with the Antec NeoHE PSU which is pretty quiet. However, there is a known incompatiblity issue with a few motherboards such as Asus. Antec has resolved this issue with a newer version of the Antec NeoHE, but I don't know the version number.
Production of Socket 939 Athlon 64 will end this year. Therefore, in the future it will become difficult to find a new socket 939 dual core Athlon X2 64. In fact, AMD may even raise the price on those as they have done with the Athlon XP when production on those CPUs ended to encourage people to switch to socket 754 Athlon 64s a few years ago. If you wish to future proof your PC you should wait for socket AM2 to come out and build your PC around that.