If we are talking games, at this level it wouldn't really matter - the graphics card would limit the performance before the CPU, so he may as well save the money. If you turned down to graphics settings so that games become CPU limited then in some the quad would win (with its extra core) and in some the X3 would win with its higher clockspeed. Its really all down to the game.
In CPU bound multithreaded tasks, the X4 would certainly be the better choice (though the OP didn't mention needing this).
But TBH, if the OP really wants to go for a Quad - I would suggest going for a X4 955. Its an extra £15 on top of the 810 - but in this case the performance crown is not in doubt.