Might be easier to just drag n drop the data on the 4GB onto the 20GB drive before you dismantle the old PC. (if there's room on the 20GB drive that is). It would depend on the size of your new case as to whether there would be room for 2 additional hard drives. If it's a full tower case then almost certainly there would be room, but if it's a midi tower then you might be pushing it. You'd need 2 free 3.5inch internal bays. Only problem is, the ribbon that connects your hard drive to your IDE port on the motherboard only has 2 connectors on it, meaning that once you've set up your 20GB drive as slave on the same ribbon as your new hard drive in your new PC, there would be nothing to connect the 4GB drive to. Depending on how many CD-ROM, DVD, and CD-RW drives your new PC has got, you could set up the 4GB as slave to one of those. If you've just got the one (DVD drive for example) then you can just connect it to the same ribbon as that, and set it's jumpers as slave. (that's if the drive will fix near enough for the ribbon to reach!) If both connectors on the ribbon are occupied (with a DVD drive and a CD-RW for example) then things get a bit harder. If this is the case then you might as well just stick with your new hard drive and the 20GB one. It's probably not worth it just for the sake of 4GB, you might be able to get something for it on eBay!