Ok I have tried both paint effects and Maya Fur and the results are below.

I have to say It was a lot easier to use Maya fur and add baldness and length maps to controll the grass near the rivers edge then it was to use paint effects, but still it needs adjusting to actually look like real grass, but I could render in mental ray with out having to conver to polygons or anything which is nice.

With paint effects it took a while for me to familiarise myself with the settings but i feel with a small patch of grass its fine, but over the hilly parts of the terrain bits start floating and it all gets a bit messy. The grass its self i think looks better than when using fur. I guess when paintng with my wacom i can set the density and length to be pressure sensitive and paint respectively in the ares that need longer and more sparse/dense grass, but that would require more time then I spent doing thease tests.

I also couldnt use mental ray for all the grass because when I converted into polygons half the grass dissapeared and when i set the poly limit to unlimited maya crashed. So only the small patch of grass i mangaged to rendered with mentalray and im not sure I like the result. The texture seems to have changed... Any ideas? :S

I am not realy sure about mib_binary proxies as I never used them before and unsure how to set it up, so any information or pointers/links would be cool.

One other thing, is it me or do my mental ray renders look slightly out of focus?