Depends on the motherboard/cpu. Some older ones will not allow you to do that but we are talking 2005 era. If its anything ddr2 or ddr3 based you should be fine.
As for using a 2+4 arrangement. Realistically you most likely wont notice any difference from a normal paired set , but technically the sizes will throw off the calls a little bit. A more pronounced example would be the NVidia gpus with 2gigs on a 192bit bus where the ram distribution is uneven. But as previously mentioned I doubt there are any realistic scenarios that it will matter and you'll notice.