See if decreasing the number of hosts to "Keep up" on the Hosts page helps.

Also, before you start Gnotella, press Ctrl+Alt+Del, look at the list that pops up, and see if there are any programs running in the background that you don't need running at the moment. Click "End task" for each of those (leave "Explorer" and "systray" alone though).

That'll free up some resources.