For graduated ND filters, I'd recommend using a Cokin holder and HiTech filters. Cokin filters are not color neutral and you won't really get the color you want. HiTech, Lee, Singh all make resin filters that can be used in the Cokin Filter System holder (or you can opt to pay a higher price for their own filter holder). Lee and HiTech are relatively cheaper than Singh-Ray and optically good. I got mine from Filter Connection
http://www.2filter.com/prices/htpackages.html. B&H also carry them. Singh-Ray can be seen at
http://www.singh-ray.com/grndgrads.html. These filters are the resin square filters. The advantage is that you can slide the filter up and down to match the horizon and achieve the effect you desire. They come in soft and hard, the difference being the deliniation of the change. Hard are good for even horizons, whereas the soft perform better for uneven horizons. They can double as full ND filters when pulled fully down across the lens.
If you just want ND filters, I would think any circular filter would work as any other filter, in choices of the amount of light reduction desired. Cheap glass will give you trouble, so I would look more towards the better known companies and multicoated filters. If you want to go all out, you can consider the Singh-Ray Variable ND
http://www.singh-ray.com/varind.html at a smooth $340 fee. I've heard of people using 2 polarizers in a similar manner to the idea of the VND filter, but I have no personal experience with that. I do have the VND and I can attest that it is a VERY effective ND filter that will control the amount of light entering the lens over a 8-10 stop range.