oh - this seems very cool

with cpu +density i get 5,151sec (and ca 70 fps)
with gpu 1,32 sec -> but only 20 fps
and a image like this:

same with gpu+weld on cpu (there it needs 12,185 sec ; 28 fps)
edit: o my specs are: Core 2 Duo E8500 3.8 Ghz (overclocked)
4gig RAM
GF 8800 GTS 512 MB (overoverclocked - with special coolingsystem XD )