Well , i have done this quite a few times . First on my old pc(Cel300) psu ( about 300Watt) and worked fine . Then i changed funs on my moderns Pc(p4-2.4) Psu (450 Watt)(it has 2) and also put some speed controllers for it , and finally on my A1200 T psu (250 Watt)i have changed it with a quiet fun and rpm controller too . No problem at all with the heat, airflow . Ofcource with this change i put a better fun that the Psu had so it is safer i quess. Also for the 20cfm you are telling i have put similar specs fan and had no problem on pc . In Amiga the need for power is even smaller so the heat also is smaller and these funs are quite enough for all . :-)
Happy modding .
