The three temperature readings that SpeedFan displays are:
idle/load
Aux: 61
Processor: 55+/65
Motherbaord: 45

i've just downgraded my case fan from a noisy powerful fan to a dead quiet silenx fan.
I've not noticed an increase in a any of the readings. which is good.

but i'm still concerned that they are too high...? thanks

Those readings seem stable to me. 65 may be a little high for your cpu though.Why don't you atatch another fan to your case?They are really cheap and it will cool things down a bit.I've just put one on and ive noticed a drop in temp of about 10 degrees.

ASROCK K7S8X M/B
AMD ATHLON 2000+XP
384MB DDR 266MHZ
GEFORCE4 AGP8X 64MB
MAXTOR 4R060L0 60GB H/D
LITEON SOHW-1213S DVDRW
SAMSUNG SM-332B CDRW/DVD
QTEC AMD COOLER NOISELES
AEROC

here are the best ways of lowering temps:

dust everything clean with a can of air

clean up your cables, get them out of the way of airflow by moving them to the sides of your case

get at least one fan in the bottom front of your case sucking fresh air in and at least one fan at the upper rear of your case blowing hot air out
-
you say that you have 1 case fan, get another one and mount them as i described above
i have the same processor as you and my cpu temp is around 37C idle and 49Cload after having followed all the tips i just listed

another way to lower temps is to undervolt your cpu
to do this just go into your bios and reduce the vcore voltage
but you may run into stability problems and to fix this you will have to go back to the default vcore voltage

what is painfully obvious to one person might be just painful to the other

Make a blow hole in the top of your case heat rasies, put a fan there and thats gonna help.

Get the best thermalright