20s startup sounds pretty good to me - I think that this part is probably OK, and your browser problems could be your DSL of course.
Other than that however, I think I will have to turn this over to folks who have an Intel Mac. I have a PPC G5 and so we can't really compare ...ahem... apples and apples here.
On my G5, iTunes was/is SIGNIFICANTLY faster than on my 3.0 GHz PIV PC. I am surprised that it is slower on your new Mac. A 2.66 GHz Xeon ought to be faster than a 2.3 GHz PPC G5. Interesting...
Have you tried looking at Activity Monitor, to see if something is eating up spare cycles on your machine? Type CMD+U to bring up your Utilities and select Activity Monitor. Poke around and make sure things look normal. If you see something eating lots of CPU when you think the machine should be idle, you may have found your culprit.
Also, I would try disconnecting your machine from the DSL, powering off and then powering back up still disconnected. How does the machine feel when it is completely off the net? This is a test for a bad network card or a bad DSL router. It is a long shot, but worth the try...