Using instancing, Agus3D created this image containing 19 billion (19 000 000 000) triangles. The kicker? The scene used only 200MB of memory and required 40 minutes to render in Cycles. The .blend file is available, so go try it for yourself!
I wanted to test Cycles performing landscapes, so i setup a quick scene with 54K instances of two tree meshes each one aprox 200K and 150K polygons and four texture maps about 512^2pixels, the blender memory indicator displayed about 200MB and the process window showed about 500MB ram so there is a bunch of memory left to put a lot more. Nice!
I predict that with 4Gbytes you could put more than 500K instances or perhaps a lot more! you could easily break the trillion polygon barrier, If you are willing to wait for the label "copying transformation to the device" i wonder if that could be optimized, because when it start to build the BVH in make it in a fraction of that time.