Well.... 8GB is a total overkill unless you play 8 modern games at the same time while editing an image at Photoshop and doing some 3D works in 3ds Max lol.
And if you happened to have a 32-bit operative system it would be a total waste of money because 32-bit operative systems can't use more than 3GB of memory. (due to architecture limitations). To use more than 3GB you need a 64-bit operative system such as Windows 7 64-bit... or Linux 64-bit (also known as AMD64/Intel64)
DDR3 memory is faster, the only problem is - of course - the higher latency.
Try to get a low latency kit (and low voltage, why not?) if you find a cheap one.
4GB should be more than enough on today's standard.