Jeff,
You may find a presentation by Tim Gorman titled "Scaling to Infinity" useful: http://www.tcoug.org/Archive/Fall2004/Fall2004.html
He describes the "virtuous cycle" which centers around best use of partitioning and the other techniques mentioned by the previous posters such as Niall. Tom contrasts the virtuous cycle with the "death spiral", which seems to be what your site is experiencing. This probably won't be any news to you, but it is often helpful for an outside expert to present the suggestions in a coherent manner. Basically your system has probably reached the limits of its scalability. You may make some small changes that will postpone the inevitable, but the point is that some significant changes must be made to have a scalable future.