Kevin, you might like to consider Hitech filters from Formatt Filters (
www.formatt.co.uk ) who do a range of filters Hard and soft ND grads and ND filters. They do different sizes, 100mm wide and 85mm wide which fit in the Cokin holders. They sell Hitech holders too. I started (in my film days!) with the Cokin P-series which are ok but only come in soft grad form. The 85mm Hitech filters fit in the Cokin P-series holders which is handy since I already had a few and the mounting rings too. My largest filter ring size is 82mm (on a 24-70 zoom) and the 85mm works fine on this lens. The main difference I have noticed is that the Hitec have no discernable colour cast...ie are truly ND, compared to the Cokin, which can produce odd casts, especially if you double them up. Formatt sell them in kits of three and individually, a set of HD Grads (0.3, 0.6 and 0.9) cost about £30 for the 85mm and £60 for the 100mm. If I were starting from scratch I'd go for the larger size. I do get some vignetting at 10mm on my Sigma 10-20, but it is ok at 11.5mm. If I need to use them at 10mm, I ditch the holder and hold the filter in front of the lens...it works fine! I have found also that the most useful grad is actually a 0.45 (a stop and a half) which gives a perfect balance for the majority of situations. They sell this as a special and will include it in the kits for a small extra charge. Formatt, I found very good to deal with....after chatting on the phone I placed an order on a Thursday afternoon, and the filters were delivered by post the next morning! Hope this helps.
Cheers
Stewart