I have 2 HDs next to each other (BTW, I replaced the IDE 40 cable you can still see in the picture with an 80 cable after reading the FAQ - and switched to DMA-5 mode!)
Are they too close to each other? Will the heat impact performance/durability?
I would seperate them if I was you. I did in my system because my hard drives where running a little hot.
Heat is the enemy and will reduce the life span of any electronic device.