thanks, I think you just saved me a 100 bucks, guess I will be aiming for a polarising filter first... and then start searching for a higher ND filter...
Have you considered a variable ND filter? They give you flexibility and again, if you buy a good one they don't significantly degrade IQ. Lee do them, Tiffen, Singh Ray are all good but very expensive - there are cheap ones but I've heard very bad reports (e.g. the polaroid ones for example) and I went with the LCW which is not quite as good as the high end ones but less than half the price. Two caveats - the degree of ND achievable varies with focal length so you have to experiment whilst using with a zoom lens and they don't work well with very wide angle lenses - they are a little thick (basically two polarising filters stuck together), the variation in ND is quite small at this end and they can produce cross shaped interference patterns if you turn them too far (easy to do at the wide end due to the above) - work well at 24mm and above though in my experience (on an a55 / a77 ie crop sensor)
A good review can be found here including the cross shaped artefact I mentioned