salvaged crystals

Hi all, I've got a spare 168 chip and I've got a load of junk kit knocking about, there are various crystals on the circuit boards, lots of 12s, a 32, a 48, and a 4-100mhz + some 'others' . I've had a read up a bit on crystals, serial, parallel, fixed, harmonics, variable, I think I understand most of it.

To get my 168 working at 16Mhz with any salvaged crystals is there anything special I need to look out for? Which would be the better type of crystal to go for? As there are the 'lozenge' H49 ones as standard on the arduino kit, cylindrical, 'glass' that are low profile smd etc. a lot of them seem to be a bit of a pita to identify exactly what they are, any pointers in finding their datasheets or ID'ing a particular type such as serial/parallel?

Am I also right in thinking that if I want to run at 16Mhz then I either need a 16Mhz crystal or something that can be tuned down, so if I can't find a fixed 16Mhz crystal I would need a variable type, then its a case of getting the right capacitor circuit built (if I can find a datasheet)? Or can I derive a 16Mhz clock from a 32/48Mhz fixed crystal? The numbers sound like I should be able to but sods law probably says I can't.

How many clock sources can I provide to the bare 168 chip, I know it has its 8mhz internal, then theres the 16Mhz it runs at, is it worth providing more than one source, or will that depend entirely on the project?

I know crystals are £1 a bag on ebay but its way more fun (and cheapest) learning about crystals by salvaging them, I get to learn how to cleanly remove smd parts with an iron while I'm at it.

Can a mod remove this double post for me? happened after I got a server 500 error posting the original.

You use a 16mhz crystal to get a 16MHZ clock for the controller. If you don't use a 16MHZ... then it's up to you fix all the incorrect speed timing loops...

Seems worth it to get the right crystal.

Tuning crystal circuits is way out of scope for the arduino/AVR clock circuitry.

If you don''t have a 16 in the junkbox... they are really cheap... and you can use a 16MHZ ceramic resonator... these are commonly cheaper than crystals.