If I install more RAM should I boot into 64bit Kernel? Does it matter?
According to this article at macperformanceguide.com, you will see performance gains (5-30%) by running graphics applications in 64-bit mode.
Note that you don't necessarily need to switch to a 64-bit Kernel, you just need to ensure that Lightroom runs in 64-bit mode.
As per Apple's web site, no.
http://support.apple.com/kb/HT4287