The Nexus Breeze 500 is a good case as long as you put in the right components. My next build will be a very powerful, but quiet PC. Because there is only the PSU fan to exhaust hot air it is necessary to choose components that do not produce too much heat. The Nexus fan at the bottom (that's a good silent fan) creates a vortex that in theory draws cool air from the room floor and exhaust hot air through the PSU, thus cooling everything in the center of the case.
This case is not for the hardcore gamer who is looking for a Crossfire or SLI GPU solution. It's really meant for a single card solution and I wouldn't suggest an ATI X1800 or X1900 series, they draw too much watts, which means they also give off a lot of heat. Sticking in a GeForce 7900GT 256MB is probably doable since they only use about 52w at stock speed (core clock 400MHz), any overclocking will mean more power and heat. The typically overclock of 450MHz should be fine. A less powerful GPU should be considered if it is going to be passively cooled.
The CPU is the next consideration. If you're thinking about a Pentium 4 then I would look at another case. P4s simply consumes much more power than an Athlon 64. A Northwood P4 should be fine, I think, and Conroe will be a good choice too for Intel CPUs. Athlon 64 runs much cooler than their competition.
A CPU need the correct HSF, I would choose the Scythe Ninja since it comes with a pretty quiet fan. Hell, I would try to run an Athlon 64 3500+, or Athlon X2 3800+ passively with the Scythe Ninja. The vortex should blow cooler air directly through the Ninja's fins. For faster Athlons I would place the Ninja's fan facing the intake fan so that it blow hot up directly to the PSU's fan.
As for hard drives, anything faster than 7200RPMs is too noisy and creates too much heat, so forget raptors. Also larger capacities means more platters and more heads which translates to more heat and noise. I would say 250GB hard drives offers a decent amount of capacity, with low enough heat and noise. Samsung and Seagate makes HDs that are usually more quiet than thier competition.
As for a motherboard, any will do, and if it is passively cooled, then it'll be even better. A Zalman heatsink should do the trick.