Well...first off....my initial thought is that 20cfm is not enough for a socket A 2600. Secondly, 28 db is a bit loud for a media center.
You might try to lap the seat on the heat sink and check that there is enough down pressure for good heat transfer. Another option is the little copper adapter plate that helped distribute the core heat to the heatsink. They were about 5 dollars back then.
For my socket A's I always used spire coolers. They are fairly quiet and very efficient at about 29cfm.
Have you ever heard of cpuidle? It has special socket a cooling features - unfornuately it's not free. Software does work, from my experience - maybe there is a freebie version of it out there somewhere.
Athlon xp is hot CPU's, i have seem many customers with overheat problems. But it always was customer fault. like putting computer in those tables that has computer shelf with door, and locking the door, newer cleaning dust clog up everything,
for CPU that idle 45-50C its easy to get 60c+ in unventilated space Athlon xp will lock up 65C+ and i would consider bad candidate for media pc
in your case normal with stock settings Athlon should run 40c+ if there is good ventilation in case. if inside of case gets hot cpu will overheat
i remember there was fake Athlon xp 3200, bad ppl took 2500 model and hardware moded it to register as 3200 but it was only possible on 2500 not 2600.
There were several mod chips out there. It is difficult to remember which ones they were but there were a lot more than just 3200s. Personally, I never came into possession of them, but I do remember many boards where people complained about the issue.
i believe the earlier athlon xps (1600-2100) were rated up to 90C max, while the later athlon xps (2200 - 2800+) were rated at up to 85C max... though you are restarting at 65C, it doesnt seem that specifically should cause it to overheat either.