If I were to run Ubuntu, W7 AND Mac OS X via VirtualBox (or Parallels) on a Mac Pro, what would I need to make them run perfectly fine in seamless mode?
And I mean, no hiccups, no laggy windows, etc.
If you are looking at buying a new Mac Pro soon, don't. The current line is outdated and will probably be renewed either at WWDC or when Intel's Ivy-bridge is announced later in the summer.
NEVER buy RAM from apple. They charge outrageous prices for it - installing the RAM yourself after purchasing from newegg will literally save you hundreds of dollars.
Finally, you don't actually need a lot of computing power to run virtual machines. On a quad core CPU and 4GB of RAM I run two linux VMs quite easily, so I'm sure that with 8GB and a 4-8 core computer you could handle a more heavyweight windows client as well.
Get the biggest CPU and the most Memory you can. Discrete graphics would help too.
There will always come a time where high-resouce applications running in any other OSes will result in lag, hiccups, or slowness. It is unreasonable to expect otherwise.
But by getting the beefiest hardware you can afford, you will minimize the possible conditions under which that would occur.