Computer Learning

[Scrooge McDuck]

Help with a heat problem please.....
For almost two years now I've been using the same system with no real problems.
Now, it has started to run a little hot.
specs:
AMD 1.3G CPU
20G HD 70% free
512 M RAM
Windows 98se OS
Counting the power supply, I have four fans, intake, exhaust, smart-fan on CPU, and power supply.
All three fans seem to be operating the same as they always have.
But, instead of the temp running in the high 50's(C) and low 60's, it's now running in the high 60's to low 70's.
If I take the side off the box and turn on the house AC the temp drops back to the low 60's.(and I freeze my azz off!)

Two questions; How high can the temp go before it does damage?
Is there any way to find out what's causing it?
(all the standard system checks and diagnostics show no problems)

Thanks for any help.