Right now, the ProWater 850i is their top kit. They sell upgrades individually:
http://www.thermaltakeusa.com/Products.aspx?C=1162

I can't find it, but there's a review online somewhere I read last week where they added a Thermaltake GPU block and the 2x120mm radiator to the mix and got better results. I guess the ProWater's pump is strong enough to take mild upgrades. The only real downside I see to the kit (aside from lemonlime's remarks) is the small reservoir. I'm not sure if it can be replaced or not.