open up the agent console, and up in the top left, click on the word "Agent". Next, click on "Properties".
In the window that pops up, find the 4 buttons on the left. Click on the one that looks like a sphere (Network). In there, you will find a couple of sliders.
For your connection.... i would set Delay Between Measurements to the minimum (15 seconds).
The next slider is a bit up to you. I have 1.6mb down and 128k up on my DSL. However, I have 3 machines running DIMES Here at home... SO I set 2 of them to 21k transfer rate, and one of them to 31k transfer rate.
That way, they take 2-3 seconds each to send out all the data they need to send, which isn't too bad.
With yours, I think i'd set a single machine upwards of 40-50k, 2 machines, 30-40k, etc.....
Next, choose default protocol. This currently has no impact on the points you'll earn. I have 2 machines as ICMP, 1 as UDP. I'd say go with ICMP as it is 'more popular'.
Next... make sure that your firewall ALLOWS netdimes traffic in both directions on the protocol you chose. If you chose UDP, make sure it allows netDIMES UDP access both ways. Etc. etc.
Also... another way to 'speed up' things is right before you go to minimize the agent to the tray (or just to keep it up as a window), to save CPU cycles (especially if you are doing another DC project such as anything BOINC based or Folding@Home), make sure to leave it on the 'messages' tab. Sure, it ain't pretty, but when you don't let it display the graphic floating pink chicken thingy and the octopus attached to it, then it CAN speed things up a bit. It'll save you 10-15% on your CPU cycles, and may speed you up a tad on netDIMES as well.