I have DSL - 10Mbps down and 1Mbps up, still works fine. You can monitor you bandwidth simply running command prompt (start->cmd) and putting in command like ping -t google.com - while nothing is going on (no porn torrents!) you should see some consistent numbers in ms all the time - try kick start GSA and see if those numbers start to flux/increase and you getting timeouts. If yes, reduce amount of threads, if no - leave as is. Just keep a bit of an eye on it.
GSA actually eats most of the threads/BW on verifying links while most CPU consumption goes to identification of keywords/etc.
I can guarantee that with your net and some 100 threads you should be fine with what you have without any affliction to remaining internet services (porn torrents

).
Give it a try.
You can always rent VPS and transfer all the stuff there. That quad system seems about right to do all the stuff.
Just my thoughts.