This might help you out a bit: Intel's ARK comparison of the two.
X3460 = Newer, more threads (but not more cores), better clock speed, higher FSB, less (but faster) cache, more instruction sets, and much better virtualisation support.
Ideally, you want two X3460's and I can't imagine the price difference would be that much more than two E5405's.This is not possible, due to what Chopper 3 pointed out below
Unfortunately though looking at a spreadsheet doesn't really tell you much about how it's going to work in real life.
Personally, I would bite the bullet and try it. If it's inferior (because you need more cores, not more raw speed) then you've got the option of adding a 2nd processor (for additional cost), or waiting until you need another server and replacing it with the server of the first specs.
I know that's not an ideal answer though. Maybe your OEM will loan you one for a week to try it out?