If your service is I/O-bound (i.e., does a lot of read/write operations to disk) you need a very fast disk, especially when using a relational database. Don't spend the money on a fancy video card and monitor! A cheap card and a 14-inch monochrome monitor are perfectly adequate for a web server—you will probably access it by telnet or ssh most of the time anyway. Look for hard disks with the best price/performance ratio. Of course, ask around and avoid disks that have a reputation for headcrashes and other disasters.
Consider RAID or similar systems when you want to improve I/O's throughput (performance) and the reliability of the stored data, and of course if you have an enormous amount of data to store.
OK, you have a fast disk—so what's next? You need a fast disk controller. There may be a controller embedded on your computer's motherboard. If the controller is not fast enough, you should buy a faster one. Don't forget that it may be necessary to disable the original controller.