Cables: unless you still have your Asus' box (which may include a set or you buy the new hard disk in a retail package, you'll have to buy them seperately. The data cable is much narrower than an IDE ribbon-cable. The power cable is either already there on power suppply unit or you must use an adapter-cable (from 4-pin Molex to SATA). Some WD disks have legacy (Molex) power connectors on the drive itself, in that case you don't need anything special for power.
Your Asus supports SATA I (SATA150), whereas most new drives you can buy meanwhile are SATA II (SATA300). You may have to set a jumper on a SATA II to run it as SATA I. The difference is the peak data transfer rate: 150 Mb/sec vs. 300 Mb/sec. Your current IDE disk(s) can do either 100 or 133 Mb/s, so the difference in speed is not enormous, but it's still noticeable.
For using it as your C:\ drive, you'll have to select it as boot drive in BIOS as chef said.
That should get you started.