To do this you need add the following lines to your .emacs file so that when you view the pdf using evince it will tell evince the page number:
(setq TeX-view-program-list '(("Evince" "evince --page-index=%(outpage) %o")))
(setq TeX-view-program-selection '((output-pdf "Evince")))
Then once you've opened your tex file in emacs you need to turn on source correlation using the command:
C-c C-t C-s
Or if you don't want to start it manually every time then add the following line to your .emacs:
(add-hook 'LaTeX-mode-hook 'TeX-source-correlate-mode)
Also, if you want to prevent emacs from promting you if you want to start a correlation server then add the following line:
(setq TeX-source-correlate-start-server t)
And that should be it.
Edit: actually, it turns out that you shouldn't use the package pdfsync, so I've removed all reference to it. It isn't needed for this solution (apparently it's now built into all TeX-based engines) and using it actually seems to cause problems (for me it seems to break code using tables).
Edit 2: I was having problems with auto-enabling TeX-source-correlate-mode using .emacs, but now they seem to have gone. Not sure if that's because I updated emacs or because I removed pdfsync. Anyway, that works now :)