Hey Matt,
I also have that problem with my gigabyte GTX 560 Ti OC in Heaven 2.1 and 2.5, but only when I run the card in the 2nd PCI slot (the reason for putting the card in the 2nd slot was to test how well a 2nd card would breathe in my future SLI setup). My mobo is SLI-certified, but only at 8x/8x, while single card in 1st PCI slot runs at 16x. Like I said, I get big nasty artifacts in Heaven 2.1 and 2.5 when I ran my GTX 560 in the second PCI slot (at 8x) rather than the 1st (a couple of times, trying to push down the clip on the PCI slot that releases the card with a screwdriver [bad idea, yeah], I accidentally pricked the mobo with the screwdriver and I´ve thought that the artifacts that I only get in Heaven (have never got artifacts in 3Dmark11 or any game played so far, regardless of which PCI slot the card´s in) could be due to a slight circuit damage on my mobo, having been pricked accidentally TWICE with screwdriver (nowadays I use the finger to release the card from the slot). Also, what is really strange is that if I put my GTX 560 in the 1st PCI slot, I can run heaven 2.1 and 2.5 virtually artifact-free, so it´s actually a problem with the 2nd PCI slot, but then again, I have run a HD4870 with a HD4850 in crossfire mode on this same mobo (also, after potentially-damaging the mobo with the screwdriver) without any artifacts, glitches, crashes or anything like that in games or benchmarks so maybe I got lucky with the screwdriver incidents. Also worth mentioning is that when I get the artifacts in Heaven with my GTX 560 (only in the 2nd PCI slot), those artifacts also happen in the DX10 version of the benchmark. My drivers are 280.26 and they don´t give me too many problems.
Also, I don´t like messing around with the clocks (I originally got a gigabyte GTX 560 SOC at 950 mhz but RMA´d it because of abnormally high temps while gaming as well as artifacts and glitches in 3dmark11), so my card is at the factory overclock of 900 mhz and pretty much stays at 69-70 ºC during heavy-gaming. In general, I´m quite pleased with the card but I also have your problem and would like to know why these artifacts happen specifically in Heaven.