Grant Fritchey (7/9/2014)
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.
Kind of missing part of the equation here is how busy is the 2K system, what kind of role does it have (transactional, reporting etc.), how beefy is the 2K5 system, has the 2K system gone through the Upgrade Advisor (detect discontinued syntax such as dump transaction etc.), is there a normal operational maintenance window (backups etc.) on the 2K and so on and so forth. We are still guessing and speculating!
When attaching a 2K database to a 2K5 server, there is a lot of I/O and CPU load involved. My rough rule of the thumb is 1 core + 8Gb ram = 50 Gb of db file size p. minute. A very rough guesstimate though ;-) That would be about 150 core minutes, my guess is that it entirely rules out such an operation within the downtime window, which is more or less in line with what Grant said.
Without the whole picture it is still guessing, one can ask if it is possible to travel 35km in a minute. It is but it takes military grade (very expensive) hardware and yet it would need a flying start!