I had a similar experience when trying to render via Lame on the Win7 family of products. I'm embarrased to say that I had inadvertently downloaded the 32-bit x86 version instead of the x64 version.
If your REAPER install folder is "C:\Program Files (x86)\REAPER" then you are using the 32-bit version. The 64-bit version installs into "C:\Program Files\REAPER (x64)" by default. The 32-bit version will work fine on a 64-bit O/S, but you must simply download the 32-bit lame_enc.dll and place it in the REAPER folder. I downloaded 3.98.2 from http://www.rarewares.org/mp3-lame-bundle.php
and it worked fine on Windows Server 2008 R2 x64.
I also installed the x64 version of REAPER and used the 64-bit lame_enc.dll attached to the sticky post. Both render my project flawlessly as expected.