Hi Terry - the 10-20 Sigma has an 82 mm thread for filters. I had one in the past but got rid of it when I changed to FX format. There are only three filters I got for it - an ultra thin UV to protect it, a circular polarizer and a midrange graduated neutral density. Depending upon light and where you place the horizon - the Grad ND and full ND's are an essential part of my landscape kit. I use the Cokin P system and often hand hold (rather than use the official holder) the filter in front of the lens to avoid vignetting. The 10-20 is very quick to vignette - an ultra thin uv and a Cokon P holder system together will do it. It is a very nice lens to use, but avoid photographing the rear of any (particularly lady) friend you wish to keep - instant tons of cellulite
I think Elements 6 will handle RAW but as a Mac man am not too sure, Don't know DPP. Enjoy the new glass it's a very good piece of kit.