Being too hard on Cycles or my GPU?

So, my problem is that when i am rendering a christmas tree (made with sapling addon) which has approx 1 million polygons with cycles, it builds the meshes (Building BVH etc.) and when it gets to rendering it just shows a checked pattern (similar to transparrency pattern) and the passes are running super fast (like it wouldnt even compute them, just skipping) and in the end i simply get a black screen. It happens only when im trying to render some complicated geometry (the christmas tree), also i cannot really reduce the polycount, as it is the central object in the scene… I guess i could make the final render with my CPU but its so slow ;D
My specs :
CPU : Amd athlon 64x2 6000+
PSU : 520 watt
GPU : GeForce 9800 GTX+

Any help would be appreciated, as i dont have that much time left, and it bugs me a bit that i get rendering errors ; )

How much memory does your graphics card have ?
Why not try another renderer like the blender internal.

oh sorry for not mentioning the memory ammount…
It has 512 MB and 256 bit
And i wanted to use Cycles because the way the lighting works is quite superb and creating materials is so much easier as well and i just wanted to get more familiar with cycles…

If with CPU the render is ok then you have a GPU memory issue. Try to delete things until it renders to better know.