I had an A500 a while back. You are likely to have airflow/clearance problems between the CPU fan and the sideways power supply if you are using a good heatsink, and the ventilation is lacking for a Tbird system. Get a new case and be done with it. Save that nice quiet A500 and make Mom a web-surfing system out of it or something.
You mentioned noise level as well as performance, with regards to the fans and power supplies. Check out Gizz0's
fan site, which has links to online cooling stores too. I recommend the Panaflo L1A as a very quiet fan, and you can increase airflow and decrease noise by cutting away any metal grillework in the case where the fan blows. Those Enermax "whisper" power supplies are supposed to be very quiet, and there is a non-whisper version too. I have one of the non-whisper ones, seems very quiet anyway.
For a new case, the Enlight 7237 is what I would get, since it is inexpensive and can be bought with no power supply. It's only going to add about $30 to the price of the power supply if you buy it from
CompuCheap, and shipping to CA from WA is not going to be too wild... or you could get them locally. The case itself comes with a Sunon ball-bearing fan, not the quietest fan around. It has four 5.25" bays, which is the next best thing to a full tower.
If you want a full tower, an Inwin Q500 would be a great buy, and unless someone is swapping the power supply, I believe the 300W is an Athlon-approved PowerMan or Sparkle. Their site is down at the moment or I'd check that... should be
www.in-win.com. The Q500 has positions for three auxiliary 80mm fans, and has 5 5.25" bays and one exposed 3.5" bay.