CPU vs Video Card upgrade?

yes, you are.
think of this as following:

you have excavator. Roughly speaking it is consisting of an engine, hydraulics, and arm with dozer(the huge metallic thing to grab dirt - sorry I'm not native english speaker). The main objective is to move dirt as much as possible.
so here goes the analogy:
engine = CPU
hydraulics = PCIe
dozer = GPU

so here is the logic: if you have weak engine, you can't load your hydraulics enough, thus can't move dirt enough (or not fast enough). If your dozer is to small but your engine is more powerful than needed, than you don't have good efficiency, thus your setup (engine) is overkill (you could go with cheaper engine and still have the same performance)... so for best (price/)performance you need balanced system.

for example   the examples in your previous post are:
Q6600 & 6950 is not good combo because the GPU will not perform to its max, thus low FPS. So you'll need better CPU to go with 6950. Good CPU for this would be Q9650 (what is in the name, eh?) while retaining LGA775 compatibility. Or any of the newer i-what-ever-quads...

The second combination (i2500/2600 & 9800 GTX+) is also not well balanced because of way too powerful CPU for that GPU. But still, I like much better because GPU in general gets upgraded much more often than a CPU (+motherboard & RAM for that matter). So having good CPU as a base is always better because of several reasons: CPU is the base/clock/heart of your system. It performs many other tasks than just feeding your GPU with data. For example in gaming, except feeding your GPU with data, it also does the enemy A.I, sound, all those windows background processes, blah-blah... you get the picture.