Yes
No.
AGP = Accelerated Graphic Card
The type of memory used by the graphics card. DDR1 is slower, DDR2 is faster. They both plug into the same slot in your computer.
graphic card
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VGA
If the mac graphic card match the Dell pc slot for graphic cards, it is quite possible.
You need to be more specific...
A DDR2 memory stick cannot be installed correctly in a DDR slot.
No, but I think you've got a few terms confused. DDR2 "slots" are for the main system memory. A "DDR3 graphic card' is a graphic card that uses DDR3 memory. A PCI-E DDR3 Graphics card should work fine in a PCI-E slot, or a AGP DDR3 Graphics card should work fine in a AGP slot (4x or 8x, dont try to use a 4x or above AGP card in an older x1 or x2 slot).
A slot card is a modular expansion card, which can be inserted into a free expansion slot in a PC computer. Most popular types of slot cards are graphic cards, sound cards, TV tuner cards and video editing cards.
No. A motherboard will specify what type of memory it supports. The DIMM slot for DDR,DDR2 and DDR3 have different footprints.