I just built a PC around the ASUS AT3IONT-I Deluxe. It's got a passively cooled CPU and a power brick power supply.
I've been running it with a single 92mm case fan and the temps haven't gone much above 20C. I suspect you could get away without the case fan if you didn't push it hard and kept...
I'm selling my 13" MacBok for one of these.
Is the CPU upgrade worth $100? According to CPU Benchmark the SU7300 is only a slight upgrade from the SU4100.