Most motherboards only support a specific subset of CPU's. If you change your CPU you MAY need to replace the motherboard... but if you do that it will include the appropriate BIOS.
You may also need to re-install the Operating System.
It's not. There are certain obstacles to upgrading the CPU (socket type, BIOS compatibility, cost effectiveness, etc...), but there are no inherent disadvantages to upgrading.
You are flashing the bios.
Download cpu-z or it might have it somewhere in the bios.
There are several web sites that will sell you an inexpensive BIOS upgrade. Just download the file and follow the instructions.
Yes
The CPU depends on the BIOS to fetch instructions for processing and then to present it to the user after it is processed, The BIOS also depends on the CPU to process the information it fetches and sends back.
BIOS of what component? your graphics card will have a BIOS, the CPU will have a BIOS. If there is nothing wrong with yours don't bother replacing it.
No, you have to "flash a bios" with the up to date version of the software from the vendors site. This would not be an upgrade per say.
The POST runs
Probably not, unless you actually change the BIOS chip.
Because it makes the CPU run hotter
yes if you have a celeron CPU you can upgrade to highter rated celeron's but to a max of 667mhz bus speeds. as for c2d CPU's again providing you keep with the 667mhz bus range the sky is the limit ! mine was a t2700 which i upgraded to a t7600 ! i have also upgraded my memory from 1gig 800mhz to 4gig 800mhz bios sees new CPU and memory correctly and set them correctly