Ah great sandbox aviator!
Is it so hard to send e-mail these days, or am I now not worthy of your consideration!!
For a start, the main deal is whether you have a TX motherboard in the old clunker. If not, you cannot even utilise UDMA2 mode, and almost anything you put in will suffice anyway, no matter what RPM (5400 or 7200).
The other problem you will have is the BIOS. On the older equipment, the BIOS does not recognise HDD's above about 8Gig, assuming your motherboard is reasonably new (less than 2 yo). Also, you will need to run Win95 OSR2, or above with FAT32 partitions if you want windows to be able to access more than a 2.1G partition.
Either of the drives you mention are of good quality if correctly handled/packed. The seek time will not be noticeable on something this ancient. Basically, it is what is available at the best price. I would stick to the main brands (Quantum/Maxtor/IBM/WD) Some of the newer ones on the market have been problematic (Fujitsu, Samsung etc) and you may have better support if anything does go wrong in any event.
If you really need the 20G, you may have to replace the motherboard. Why not look for an 8G (if you are lucky). I am sure she will not need much more than that, it hold a lot of e-mail!
BTW, love the bit about the better half, bet you never say that to her face.
Cheers
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When all else fails, read the manual!