Roger Miller,
You can put a new Western Digital 7200rpm Drive as your new Master with the 5400rpm drive as the Slave.
I would suggest going with the Western Digital 80Gb 7200rpm w/8Mb Cache WD800JB.
Windows is all about Disk Access and you want the fastest drive you can for running Windows. The JB Series of Western Digital Drives is one of the best performing IDE Drives you will find.
Last thing, I mentioned the 80Gb vs the 40Gb since there should not be much of a difference in price between the two. The 80Gb is a better value for the money. The difference between the two in the U.S. is only $7.75 (40Gb $63.25 and 80Gb $71.00).
Take Care,
Richard