ok, well, i do in fact have nVidia, however it is an older system.  so i guess my question would be this:

if you can render using GPU, can you not also render using CPU... i mean isn't CPU simply a default kind of thing?  i don't mean necessarily the mixing/potential overflow type thing, i just mean doesn't one always have the CPU option?

Not necessarily.  For example, let's say you have a 2mb GPU.  You choose to render GPU, the scene you are trying to render overflows the 2mb on the GPU and your system just crashes in some of the rendering programs.  Once you run out of GPU memory, you're kinda, sorta just screwed.  LOL!