I'm about to start building a new PC (when all the parts arrive have arrived). I'm using a mini-tower case that has the PSU on it's side. The 120cm fan on the underside of the PSU will face the motherboard, right above where the CPU goes. I'm using a retail Athlon 2600 Barton (ie. it comes with an AMD fan). I think the CPU fan would normally blow onto the CPU heatsink, but as it will be right below the PSU fan that is blowing in the opposite direction, would it be ok to reverse the direction of the CPU fan so that it blows hot air from the CPU into the PSU and out?
I haven't actually got the CPU yet. When it arrives I might find it doesn't fit in the gap under the PSU with it's fan on, in which case I'll have to resort to plan B (buy a new, bigger case instead of using an old one).
It's really a tough call, it might work, but it might not. There are a couple of heatsinks out there that do use a fan blowing away from the heatsink, but not many. Also, I'm pretty sure this would void the retail pack warranty.