I don't use my Antec Aria for my Knoppmyth box (it's actually my gaming rig!), but I know many out there do. I had a hell of a time finding a CPU cooler that fit under the power supply. I have a Zalman flower-type cooler that fits under the PSU, but on my motherboard (MSI KM4M-V), the CPU was too close to the edge - I couldn't get the side panel back on the case! Ultimately, I finally got one that does the trick:
http://www.newegg.com/product/product.a ... 6835114011
It keeps my CPU (XP2500+) almost 20 degrees cooler than the stock AMD cooler, and is noticeably quieter as well. I don't think the fan itself is that much quieter, but it keeps the PSU fan in the low RPMs. And it's cheap!
I think the main factor in the decrease in CPU temperature is that the heatsink/fan are so low-profile that there is more room for the hot air coming from the heatsink to dissapate.
Next step: put a quieter fan in the PSU!!!
Good luck keeping those Athlon XP's cool!