"SCSI, like a Porsche, is fun to drive, and worth mucho bragging rights to anyone familliar with it."
actually SCSI is MORE then a porsche.  you use it all the time.  a porsche isn't used all the time (unless you want it stolen, or just don't care if it gets banged up).  SCSI, on the other hand IS useful all the time in the speed you get.  you'll feel it instantly.  it's more then bragging rights, its one of the best tweaks in a computer you can buy.  think about it, you're hard drive stores ALL your data.  would you want a slow drive?
sure you can sometimes have a slow piece of RAM, without feeling a difference at all, but with a good hard drive, you almost automatically see a difference.  if it wasn't for hard drives being way too slow for CPU's, you wouldn't even HAVE RAM.  
hard drives access times are measured in milliseconds (thousandth of a second).  cpu's are now measured in fractions of a NANOSECOND (1 ghz has 1 tick of the clock per nanosecond).  that's a BILLIONTH of a second.  in between the two are MICRO seconds, which are millionths of a second.  you get the picture?
hard drives are one of the slowest components in your computer.  anything you can do to improve their speed (in access time especially) will greatly effect your computer's speed.  
EXCEPT if you begin to have your slowest bottleneck being measured in the same unit as the CPU (ie RAM), then the returns you get from speeding it up, whether access times, or raw data transfer rate, is not as great as the increase in price.  RAM is the device I'm talking about for you who didn't catch on.  it deals in gigabytes/second, and is rated in nanoseconds (10 nanoseconds for 100mhz SDRAM for example).