I get them too. PC spec:
AMD X2 4800+, 2GB RAM, 2.4TB HDD (yes that is TB, not GB!), and 2 x nVidia 7900GT in SLI.
The black boxes happen usually where there's water or a gorge healspray - using latest nVidia BETA detonator drivers, it happened also on the drivers that shipped with my cards, which were WHQL official drivers.
This sucks, but it's still just about playable - haven't found any workarounds for this yet
Edit: And I also get the strange crosshair problem too.