I didnt realise the rpm had anything to do with the cable? surely this speed is built into the hard drive and it isnt powered by the ribbon cable. The rpm doesnt govern the transfer rate. Can someone confirm if this is true as I was obviously misinformed somewhere?
Why just try running the 5400 as a secondary master rather than a primary slave? obviously if you've only got the two IDE slots on your mother board, you will lose your CD drive or anything else in the second slot. Just an idea though....
A HDD controller card such as those made by "Promise" will give you an extra two EIDE controllers when fitted in a spare PCI slot. Just be aware that Hard Drives on initial startup take about 1.5 amps from the power supply. You can arrange to stagger the startup times with multiple drives but it may also be advisable to uprate your PSU unit to 450 watts or even 550 watts.
Sorry SEASHANTY, I ran three drives off a Promise IDE 133 PCI card on a system that only had a 250 watt PSU. I would agree that something akin to 350 watts would be more realistic but 550 is over the top.
I do concur with your point about spinup drawing more power. This can be as much as twice the normal running power requirement and does place quite a strain on the PSU. A 7200rpm can require up to 24 watts on spinup.
What has been missed in all of the above is the extra heat that the 7200rpm drive will create and this will need dissipating. The web is littered with stories of new 7200 drives failing early - simply because additional disk/case cooling was not considered. These drives get very hot and should not be placed adjacent to another heat source without extra ventilation.