If I were a betting man, I'd say it's the uber stuff. That killed me the first few tries for me with IRAY. I wasn't using GPU and we are talking hours to get fuzzy pictures on a 4ghz machine, lol.

I would start out deleting any unnecessary stuff, like any atmosphere related stuff. Just load the props. Maybe start with just one big prop. Select each of it's textures in the Surfaces tab and then find the Iray uber Base shader which should be pretty easy to find (I've moved mine, so I have no idea where it was hiding originally). Do a render, then keep adding pieces. I suspect you'll either find the culprit or you'll end up with a complete scene that renders reasonably fast. If the water shaders or some of the more complicated things don't translate nicely using the default iray shader, you will have to learn how to fix them yourself, or maybe like someone said earlier...there might be iray specific versions of the model or it's shaders. Of course, if this is a Poser model, it's definitely not going to be IRAY ready out of the box.

Laurie makes another good point about displacement. You can easily end up with a model that simply won't fit in your GPU's memory if you turn up the # of subdivisions really high (sometimes you have to get them high to reflect the underlying texture's pixel density)...really high isn't really a big number, necessarily. When that happens, it jumps doing to CPU only and can take forever. I'm going to guess this is related to the parameters, subdivision Level, but it's been a while since I played with that... Anyway, I doubt anyone would release a product set high enough to overload a 4 gig card out of the box and it doesn't sound like you changed things...but, it's a good point to be aware of.