How to recognize CPU bottlenecking?

After reading some threads about CPU bottlenecking I know for a fact that my CPU is bottlenecking my Video Card.
The question is, how can I tell without actually upgrading my MoBo and CPU and then running tests? Maybe there is no way except to just “know” that my pos CPU is holding things back.

And…
How much of a difference will I see in upgrading MoBo and CPU?

Current RIG:
Asus P5Ad2-Deluxe
Pentium 4 -2.8ghz OC’d 3.6ghz (Thermaltake Liquid Cooler)
1.5GB DDR2
ATI HD3850 OC’d 755/1055
3dmark06 = 6k


Thanks!

Is your CPU a single core?

You can tell if it is your cpu lagging in games if you get hitching, or if your fps has a large range ( like 5-70 fps). Another way is comparing sdmark results like Lost said.

Cfvh599, no Pentium 4s are dual core. The only p4s that are close to dual core are the ones with hyper threading, but it is still different from real dual core.

If you have the abillity to play around with your resolutions on your monitor one simple way is to see what you are getting in a game at a high res, Run Fraps to check your FPS.
If you now drop the res to say 800x600 then your FPS should increase if they dont then you are being bottlenecked as what happens is that the CPU does a lot of the physics ie where stuff is on screen when stuff is moving about.
When the CPU cant keep up with the amount of info the GPU is throwing at it you get the bottlenecking effect.
This is a very simple test that may or may not work it all depends on how hard you can work the GPU in the first place.
Start max ress AA etc and then drop the res if the FPS dosent go up then the CPU isnt keeping up.
Mactronix

your memory is your bottleneck. A 3.6GHz P4 can crunch some decent numbers if you can push the memory bandwidth up to around 6GB/sec. I have a hunch your memory bandwidth is about 2~2.5GB/sec, which is abysmal. The "1.5 gigs" is usually a dead giveaway that says "I'm not running in dual channel mode."