Greetings:

Personally, I'd use clustering technology instead of round-robin. Rotating DNS can lead to a whole host of problems, and was primarily used before modern clustering technology came up to par.

Although, we're talking about 400 uniques per minute here? You could easily handle that on a Sun box running Solaris. If you want to really nail it down, set it up on 2 Sun boxes, one for the httpd and one for whatever db you're running.