Well, I remembered using Konqueror (I believe is from 2.x or early 3.x) on a Pentium 3 500MHz I think with really few RAM and haven't found it unacceptable for daily use. But anyway, probably you need an even lighter choice if yours is even older hardware.

Then probably you don't need to try Krusader either. It is geared towards more advanced users and it won't be a lot lighter in resource usage than Konqueror in my opinion. It has an interface that is more like Midnight Commander than Konqueror, so you may not quite like it (as I don't either).