Really improved my network performance with my USB/DSL modem..
As a unix&network-dumbo I discovered some more ways to tune this:

First there's an easy way to find out what your MTU-setting should be.
just ping a preferably nearby adres (like your router adress), and do this a couple of times while adjusting the 'packet size' of the pings each time, to find out the point to when the packets get through or stop coming through.
You can do this for example like this:

In the terminal type:

ping -s 1500 x.x.x.x

where '1500' is the packetsize and 'x.x.x.x' should be the adres to ping (try using the adres you see in System Preferences > Network > TCP/IP-tab next to 'Router').

Now when nothing happens, the packetsize is too big. Exit the ping-command by pressing 'control-C' and repeat and lower the packetsize:

ping -s 1495 x.x.x.x

Keep doing this until something does happen; when you see ping's being recieved (a list appears).
When the pings get through, you can raise the packetsize again, but with much smaller increments...the point is to find the exact maximum packetsize value which get through.

This will be your ideal MTU setting....for me it is 1476.

Second:
Next to the ways discribed above to implement this setting, you can also download & use the 'BroadbandOptimzer' script (http://www.enigmarelle.com/sw/BroadbandOptimizer/) which tweaks some other networksettings (more info in another hint on this), and insert the ifconfig-line from the original hint above in the BroadbandOptimizer script.
Once installed it should be here:
/System/Libray/StartupItems/BroadbandOptimizer/BroadbandOptimizer
you can edit this using a texteditor while having root-acces.
Insert in my case

ifconfig en1 mtu 1476

just before where it says "echo "Network settings......."".

This way the BroadbandOptimize-script is your 'Network-tweak' script, and you've got all the tweaked settings in one place.

GREAT NEWS. Well, it's not that exciting. But I just upgraded to Panther and needed to set the MTU again. Nothing has changed, just use the same fix as stated here, probably all of them work, and you'll be set.

So now this works in 10.1, 10.2, and 10.3 (pre-release at least).

Is it necessary that I put this line in?
ifconfig en1 mtu 1476

Or can I just download and install BroadOptimizer and leave as is?

I thank you a lot for your tip about sending pings to find the right MTU.
I have another problem, I made a stupid mistake, maybe you can help me to solve it: I erased the MTU folder from the Library folder thinking it was the MTU folder in the StartupItems folder, Is that serious? How can I rebuild it? (I'm using Mac OS 10.2.8).
Thank you...