Javascript Memory Usage & Debugging in Chrome / Firefox (Minefield)
Have you had a chance to check out Speed Tracer yet? That should give you more of what you need I think.
With memory usage, you probably want to do a heap profile in the Chrome developer tools under the "Profiles" tab. You can compare multiple heap profiles to get a look at memory usage.
According to http://blog.chromium.org/2011/05/chrome-developer-tools-put-javascript.html you can actually use the performance.memory window property within your Javascript to check the effect of sections of code on memory usage.
This looks useful to me as interpreting the output of heap snapshots isn't straightforward.