Is it a problem if kernel_task is routinely above 130MB on mid 2007 white MacBook? In trying to get the best performance possible out of this old guy, I've been looking into Activity Monitor more and more. kernel_task is usually above 130MB (142.9MB right now). Is this abnormal? If so, what could I do to fix it?
Short answer:

Probably not, but without knowing more details about the machine I can't tell for sure. 

Long answer:

The single largest block of ram in the kernel are the tables used for keeping track of memory permissions and VM mappings. For every 4KB of ram in your computer the kernel needs to keep at least 64 bytes of data to track what process is using it, etc. Consequently the amount of ram the kernel uses is directly proportional to the amount physical memory you have installed. On my Mac Pro with 48GB of ram the kernel routinely uses over a 1GB of ram, and that is entirely normal normal.

I am guessing you have ~2-4 GB of ram, which would require 32 to 64 MB for just the tables, and the rest is in the ballpark for what the kernel needs for memory buffers, mapped text and drivers, and basic operations.