Oblivion loads data almost constantly. Every time you pass through a door or gate into a different area, the game grabs your new surroundings from the hard drive. If you're running an older (but very common) 5,400RPM hard drive, you'll wind up watching the loading bar more often than you swing your sword. We highly recommend replacing that old IDE drive with a new Serial ATA drive (usually just called SATA), which is the latest standard. If you're not sure which kind of hard drive you have, just open the side of the case and look. If the HD has a two-inch wide ribbon cable attached to it, it's an older IDE drive. If it's got just one thin cable coming out of it, you're already running SATA, and Oblivion should be just fine.
If you're ready to go SATA, we love the 10,000RPM Western Digital Raptor series of speedy SATA hard drives. Recently, the smaller 36GB drives have seen their prices dip below the hundred-dollar mark at retail. Some motherboards include SATA support already (especially the more recent ones), but if not, you'll need to add a SATA controller card for another $25 or so. (Save the older drive for bulk storage; now that you don’t have programs on there, it should have plenty of room for backing up your photos and music.)