Are you sure the other GTX 570 was faulty? Did you change the power saving setting in Nvidia control panal? On my cards if I have it on adaptive performance the GPU usage nevers goes up, so I have to select perfer maximum performance (or something like that).
lol. to me 85% gpu usage is equal to the game utulize the gpu very well or 100% utilizing the GPU . just like matto17secs said you should care more about your FPS not the gpu usage. different games will utilize the gpu differently. for games that rely heavily on GPU then 80%-99% gpu usage is the norm. but for the ones that rely heavily on cpu they might use less. as you for your dropped frame rates in dead space 2 i think that is because in open area there are more stuff to render.combine with light, shadow and draw distance all this will add more to calculation for your gpu hence drop in frame rates is to be expected. but 110fps is still way more than acceptable in my book