The recent day after thanksgiving sale has come and gone, and although I didn't try to get the Western Digital 160 GB hdd for $30, I did get to thinking. First I thought, maybe the 6 a.m brawl would have been worth it. Then I was wondering if I could have used it anyway. Someone I know once tried to add a 20 GB to an old dell machine and it only recognized it as 8 GB. Right now I have a Dell Dimension 4100 and I wasn't sure if I would run into the same problem. I figured if I would, it would be a motherboard issue. The dell website tells me this about my motherboard "CARD (CIRCUIT), PLANAR (MOTHERBOARD), NO SOUND, DIMENSION, 4100, A." I tried to figure out if that would limit me in some way, but I have no idea. Any answer?

Old motherboards (their BIOS) don't recognize larger hd's. Very old don't support larger than 8gb, some newer, but still kinda dated don't support 160+GB ones.
There are several things you can do depending on your OS (Win, Linux, other), but the best is to update your motherboard's BIOS. (t'is kinda dangerous though).

The guy that I mentioned who had the trouble with the 20 GB hdd still uses that computer. I have this computer science internship and we have recent BIOS versions on diskettes. If I take one to him to update his BIOS, will it possibly finally recognize it?

The BIOS diskette must match the motherboard model, as for a larger hd, if the manufacturer has included that feature in the update. 99% it has.
Best thing to do is to go to the manufacturer's website and download the newest update for the particular mobo model.

There is also the option that if you get newer Hdd drives for older machines, that you get yourself a PCI controller, something like a PCI Raid card usually can handle drives normal as well as being set into RAID mode. Since the PCI handles the drive it shouldn't have the problem of not detecting the fully available amount of harddrive space.

The older machine has a AL440LX motherboard. I checked all the BIOS updates I could find, but none of the descriptions seem to indicate that they help recognize hdd sizes. Intel has stopped providing anything new for it, so I guess that won't work out. I'll look into that Raid card though. Thank you both