Looking for a Hard drive at $40.

Cost is most important.
Low Noise and good performance are important.
Capacity is least important.

Will be the main OS / Boot drive for Windows XP 32bit. It will be the only drive on the system. This will be a HTPC / Office system.

WD5000AAKS, WD5000AAKX, and WD5000AADS interest me, although I hear the WD Green drives are not so good performance-wise, and it does break my budget.

WD lists the dBA levels as 28 dBA Idle for the Caviar Blues and 21 dBA Idle for the Caviar Green. Are these reliable?

Low idle dBA is important to me.

TIA

Those idle sound levels from WD are not accurate, especially considering the fact that they don't state the distance at which they measured from. I use a WD5000AAKS as the primary drive in one of my PCs and is inaudible beneath the sound of the fans in the NSK3480 housing it. The seek noise is quite audible with AAM disabled, but very quiet and muted with it enabled. The performance is subjectively quite good, although nothing incredible.

I've been looking at the Samsung HD502HJ

Which is more quiet and more reliable? The Samsung HD502HJ or the WD Caviar Blue WD5000AAKX/WD5000AAKS?

Purchased the WD5000AAKX. Installed in a very standard $40 metal case, no suspension, no isolation. Just mounted onto bare metal. I'd say it's fairly quiet. From one meter away and further it's hard to distinguish the HDD noise from fan noise. I have one 80mm case fan, one standard PSU fan and one stock AMD CPU fan. From 2 cm away, it's still hard to pick out the HDD noise. For $40 shipped, it's pretty quiet. Not silent like and SSD, one can still pick out the metal spinning noise from 1 meter if you really concentrate. But a good value for an HTPC.

If you have adequate cooling in the area (as in a fan in front of the HDD or your HDD is directly in the path of the intake air) you should consider implementing some type of isolation on your HDD mounting. Grommets can help, but even some small pieces of soft foam or felt wedged in between the HDD and the cage will help reduce any vibrations being transferred.

This applies to pretty much any source of vibration, such as a CD/DVD drive or PSU.

My WD5000AAKS is the only audible component in my system. It sounds a lot like a fan at idle, so I had to disconnect it before I figured out it was the problem. That said, it's not horrible; just blatantly audible.

I won a HD502HJ, and its not silent, but its very quiet at least to my standards, more than any hdd i have used, i have no experience with the WD Caviar blues to compare it. Its been runing for almost a year now, and no issues, i bought it as the Samsung F3 was one of the fastest hdd of its time rivaling WD blacks, i believe is a single platter so my decision went to it because of this, and i dont regret it, its been a good drive.

One alternative is could be go into 2.5 drives, since size is no importance to you, i have a Hitachi 7k500 on my laptop, and also very quiet drive (not silent), probably more than Samsung.

You are right. I just bought a Samsung F3 for my parents- it's super loud, by my standards. It's not seek noise it's the idle noise- a pervasive metallic ringing noise that I've seen on some 7200 rpm drives. Would not recommend.

How about an SSD - Newegg has 8 GB SSDs for $40. You said that capacity was least important.

Now that you can get 60GB SSDs for under $100 I doubt that I will ever buy a mechanical hard drive for a desktop - just for my server which is hidden in my basement.