The idle speeds you described sound perfectly normal. The shaking you describe could very well be motor/trans mounts, and those could also explain what feels like a "miss" to you. In any case, you might as well check all the vacuum lines to make sure they aren't rotted. 12 years is a long time so it's probable that some vacuum lines need replacing, especially the ones going to the vapor canister behind the front bumer, passenger side. Just trace all vacuum lines coming from the engine and any that are brittle, replace.
Also, it would be a good idea to clean you MAF sensor if you have never done so. Instructions for this procedure can be found here
. Afterwards, flip open the fuse panel under the hood, pull the #15 fuse, wait a minute or so and plug it back in. This will reset the PCM and allow it to relearn the engine's operating parameters.
Have you changed your fuel filter in the last 15k miles? If not, replace it.
Have you gapped the plugs correctly? The spec is 0.054", and if it's not right your engine could "stumble" a little at lower RPMs. Also, bad plug wires could cause this as well. Do the wires have more than 100k? If so, replace them.
Is your car overheating at all? Does it run warm? Does it spew white smoke at start up or at all? If you answered no to the above, then chances are you don't have headgasket problems. It is much much cheaper to do the headgaskets yourself, and in all honesty it's not too bad of a procedure. It's just quite time-consuming your first time around.