If I install more RAM should I boot into 64bit Kernel? Does it matter? I've got an Early-2009 Unibody Mac Book Pro 17".

It came with 4 GB RAM and by default it boots into 32-bit Kernel.

I've just upgraded to 8 GB RAM and my main concern is getting maximum performance from my 64-bit version of Lightroom 3 (I scan negatives and edit large tiff files).

Now that I've got 8 GB RAM, will booting into 64-bit Kernel give me any performance boost on Lightroom (and even generally)?

Should I see a performance boost on 32-bit Kernel with the additional RAM?
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.