Anyone ever thought of ideas like leaving the top cover open and leaving 1/3U gap of space to solve the thermal problem?

On the racks I've seen, the holes are not evenly spaced, so 1/3U spacing could get interesting. In addition, leaving the cover off doesn't do much to conduce airflow, so you will probably just delay the same problem.

Just go with 2U boxes unless you really, really need more than 45amps per rack.

Removing the cover is only going to make things worse. A well thought out forced airflow is going to get rid of heat far more effectively than taking the cover off.

What you might consider is cutting a few blowholes in the cover in hot areas and sticking some extra fans in. I personally like playing with my power tools :)

Any sugguestions for cutting holes and placing fans in a 1U AMD XP server?

A lot of people have shared that they have witnessed or heard of AMDs overheating in 1U casings. Were they refering to just the thunderbirds and Durons or does that include even the XP ThoroughBred-B and Bartons?