I have 3GB Corsair Dominator xms2 1066MHz DDR2 on my pc. I am thinking of going to Windows 7 and so will need more memory. I am thinking of buying DDR3 but don't know if my mother board will even accept it. I don't mind if it does not use the ddr3 abilities, sometime will upgrade the mother board.

This is the board: EVGA 680i A1

So, it it work it at all to buy DDR3? Will it fit on my board?

NO it Appears that board is DDR2.. DDR2 and DDR3 are NOT Backwards compatible.. the slots Notches are different..

You could get 8gb DDR2..

No, that board won't work with DDR3.

The physical slot of DDR3 is different to DDR2 (as in it won't physically fit), plus the chipset on the 680i would not recognize it. If you want to keep that board then you'd need to buy more DDR2 for it. If you have 3 1GB chips (which is at a slight disadvantage, as you aren't running the memory in dual channel pairs) then I guess you could keep 2 of them and then spend something like $60 on 2x2GB DDR2, i.e. bring you up to 6GB fairly inexpensively. It's usually good to match speeds of the memory, or the four-slots will default to the slowest speed or potentially cause an issue (usually in a mixed pair though).

Hope that helps.