No. Different iMacs require different types of memory modules. Most major Mac-friendly RAM vendors should be able to recommend the correct RAM for your model iMac. Please see links below for more details.
A new iMac comes with 4GB of Ram installed. You can add more at any time.
It does everything any other computer will do.
It runs on the OSx operating system. It would not be an iMac if it ran on Windows or any other system.
Sure, just copy them to a CD or Flash Drive.
There are apps that will only work with Mac OS X which can be on any Mac computer - iMac, MacBook, Mac Mini etc.
Yes. DeSmuME does.
You're computer will require a "DDR2 SDRAM DIMM". DDR2 SDRAM is the type of memory you are after, it is also a DIMM. Most modern memory are all DIMM's (Dual inline memory module), much older ram were SIMMS (single ingline memory module). DIMM refers more to the physical make up of the module (DIMMS have contacts on both sides of the module). So any DDR2 SDRAM that you buy IS a dimm. You may want to look at the wikipedia page about DDR2 SDRAM at http://en.wikipedia.org/wiki/DDR2_SDRAM
Yes. DDR2 modules are backwards-compatible. Any module up to 1 GB will work, although the module will not run at its full speed.
The same thing any other computer was used for.
Yes, but you may need an adaptor.
Turn it on by pushing the power button on the back left bottom of the iMac.
Any type of memory can be considered "tall" memory. This just relates to the physical dimensions of the memory module. So it is higher from the motherboard than similar memory modules. The main concern here is the heat sink for the CPU. When larger CPU heat sinks are used, they often cover the memory modules, so you would need memory that will fit under the heat sink. If there isn't enough clearance, then you cannot use both that heat sink and the tall memory modules.