[ESS] how to make ESS open 64-bit R instead of 32-bit R?

[Christopher W. Ryan]

I'm running:
Windows 7 Professional, Service Pack 1
on a 64-bit machine
R 3.2.3
emacs 24.3.1
ESS 13.0.9

The first part of my environment PATH reads like this:
C:\Program Files\R;C:\Program Files
(x86)\ImageMagick-6.8.9-Q16;C:\Windows\system32;C:\Windows; . . . .
and nothing about R appears anywhere else in my path.

My user path reads like this:
C:\Program Files\R;C:\Users\ryanc\AppData\Local\Pandoc\

When I open an .R or .Rnw file and execute a line of code, ESS opens up
my 32-bit version of R.  Usually that has been fine, but now I have
projects that need higher memory.limit(). I'd like my 64-bit version to
open instead.  How can I accomplish that?

Thanks.

--Chris

-- 
Christopher W. Ryan, MD, MS
cryanatbinghamtondotedu
https://www.linkedin.com/in/ryancw

Early success is a terrible teacher. You’re essentially being rewarded
for a lack of preparation, so when you find yourself in a situation
where you must prepare, you can’t do it. You don’t know how.
--Chris Hadfield, An Astronaut's Guide to Life on Earth