For rarer 5 download sources sounds about right, for SR, 30+ depending on what it is.
To maximise sources, make sure you have your number of connections set around 1000-3500 depending on your CPU power [i have an athlon xp 2400 and have it set to 3500]. Get on a large server, i'm normally on razorback:
ed2k server:
195.245.244.243:4661
but these are good too:
ed2k server:
193.111.198.138:4242
ed2k server:
193.111.198.139:4242
ed2k server:
193.111.198.137:4242
ed2k server:
66.227.96.151:4661
If you've got a firewall, make sure it's setup right. If your using Zonealarm you need to be shot
Disable internal firewall, make sure source exchange is on and if your having trouble locating sources, hop to another big server. Low ping / high user is the combo your after.