If you have Gigabit networking, then you running at a speed of 1,000Megabits/s (not Gigabytes or Megabytes which would be approximately 8 times faster -- 8 bits to a byte).
My guess is if you are using cable, then your download speed is about 10Megabits/s (T1 = 1.54Mbs). So your bottleneck for Internet is definitely not your network and probably never will be even if you were running at 10Megabits/s.
When you say you are noticing a "real slowdown" what are you referring to? Internet speed or speed to your database. I am not sure how users using the Internet would slow down your network speed or getting AC to the database. Even though you are sort of running client/server, each client is accessing the Internet directly via your ISP's DNS and not your "main computer."
You probably won't notice any difference in speed by isolating the computer. But, you will have more reliability and way less chance of a user doing something to your database or something else, not to mention requiring a reboot for something.
