don't forget that the Pentium 1 probably still has an AT case, while all Pentium 3 motherboards are ATX.
Of course you will have to change your memory banks to, as they are to slow or still edo's.
Your videokaart should be replaced by an AGP, because it would be a shame to lose performance on that area!
so you change your case, proc, you mobo, your memory banks and your videokcard...
what have you got left? just sell the old pc as one working unit, it's the only way to still get something for it!