Most current motherboards are jumperless, as some other people have said. That's not to say that there aren't jumpers on them... but for the most part, the jumpers can be left in the default position (as people have said)
As for CPU multipliers, this is the way things are currently:
AMD
-Athlon 64, s754 Semprons, Opteron: Downwardly unlocked
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Mobile Athlon 64: Completely unlocked
-Athlon FX: Completely unlocked
-Athlon XP, Socket A Semprons: Completely locked (the older ones were unlocked or unlockable; AMD has since started locking them and sealing the bridges so it's difficult if not impossible to unlock them)
-Athlon MP: Same as Athlon XP
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Mobile Athlon XP: Completely unlocked, and it will work in a desktop AXP board; because they're unlocked and have low default vcores, they're great for overclocking- both of my 2600ms do at least 2.6GHz on air cooling; one does almost 2.8... which makes it equivalent to about a 4GHz P4

Durons: Same as Athlon XP
Intel:
-Pentium 4 Extreme Edition: Completely unlocked (I think)
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ALL other desktop P4s (Prescott core, Northwood core, Wilamette Core, etc.): Completely locked, not unlockable (the exception being Engineering Sample P4s which sometimes show up on EBay or on forums... they're completely unlocked. They're also expensive as all hell because an unlocked P4 is ideal for overclocking)
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ALL P4-based (S478, LGA775) Celerons: Same as P4
-P4 Xeons (socket 603, 604): Downwardly unlocked
-Pentium M (aka "centrino"): Completely unlocked (I think- It HAS to be downwardly unlocked for speedstep to work... I'm not 100% sure if it's upwardly unlocked or not)
-Mobile Pentium 4 (all versions): Same as the Pentium M
-Pentium 3: Completely locked, except for the ESes, just like the P4
-Pentium 3-based Celeron: Same as the P3
-Mobile Pentium 3, mobile Celerons (both P3 and P4 based): Same as the Pentium M and Mobile P4
-Pentium 3 Xeon (slot 2): No idea, probably completely locked, but it may be downwardly unlocked
-Itanium, Itanium2: No idea. The Itanium isn't a consumer level CPU, and it's slower than all hell anyhow. It's also insanely expensive. And it won't run a regular version of Windows or Linux. So it's really irrelevant.
Via:
C3- Who knows, who cares? The C3 is so damn slow that it's of no consequence to anything.