I'll come right out and say it. No, it's impossible to just migrate your database over using any straight method.

I agree with the steps Hanshi laid out for a multi-step migration.

What you could do is go one step further than what Hanshi suggested and setup log shipping so that you can have, at most, maybe 2-3 minutes of downtime. You ship the 2000 server to the 2005 server, then you shut off access to the 2000 server, get the last log backup, shift that over, take the 2005 server out of log shipping and bring it online. The downtime involved is extremely minimal. If you script it very carefully (and test it to be sure), you can actually have no more than 10-15 seconds.