If it is difficult to split the nodes database between two machines--though maybe a split along the lines of nodes older than (say) the last 10,000 could be acheived?--then how about splitting off transient data from the main machine to another. Stuff like the CB, Other users, Tick-Tock nodelets and possibly some other stuff could be embedded in IFRAME tags much as the advert is now and the load drawn upon another machine without much in the way of changes to the existing DB and few (I think) changes to the HTML.

Even the preview pages might be offloadable in this way with only the final submit being back to the main site for storage?

Just another random, based-on-guess-work thought.:)