Thanks for the comments about the Article. Glad it has been of use.
Id advise against a screw in graduated filter as the are very ackward to use (in the fact that the position of the filter cant be adjusted up and down) which when composing a picture means that the horizon will have to be placed wherever the graduation on the filter.
Id recomend getting the square type either cokin or lee.
The Sigma 10-20 will make a more than suitable lens for sunrise/sunset images, the only problem is that the cokin p series type filters will only work down to around 15-16mm, below this the images will suffer from vignetting as the lens will see the filter holder.
To overcome this you will need to use 100mm wide filters such as the Lee system, which unfortunately dont come cheap. So the screw in round type looks a better option, but then has its problems as mentioned above.
The only other alternative is to get a Lee Holder and get some Kood filters, the holder is around £50 and the kood filters can be purchased for around £17. Though i havent used the kood filters, i have heard that they are good.
Lee filter holder http://www.warehouseexpress.com/buy-lee-foundation-kit/p1010393
Kood 100x125mm filters (Z-pro) http://www.ukcamera.com/denton/kood.htm
Any other questions please ask.