I'm really curious of some opinions on the differences between the two 21.5" iMacs: NVIDIA vs. ATI graphics processors. What are the benefits/pitfalls of each? I'm not a gamer, but I do use some scientific graphics programs occasionally (data visualization and molecular rendering), for what it matters. I have had poor experiences with my current MacBook (2007) with the Intel shared memory graphics processor, namely: some seemingly minor graphics jobs like a 2-3 minutes youtube clip heat up the CPU and the fan kicks in heavily and annoyingly. I don't know if I can attribute the heating up to the shared memory setup or not, but I'd really like to avoid a similar problem in a new iMac. So, which 21.5" iMac should I get?!? Thanks!