70 degrees is high for a CPU temperature, and I wouldn't play any more games until you fix something. 48 degrees C idle is a bit high, too. Pentium 4s are on the hot side, but 70 is too hot.
A) What should the optimal temperature be for a P4 3.4ghzHT processor?
It depends, but IMO should be under 50, preferably under 40.
B) Is there such a thing as being 'too cold' as well as too hot?
Not that I know of. I remember a while back seeing a video of some people using liquid nitrogen to cool a CPU and overclock it to well over 4 GHz.
C) What is a good, reliable, near noiseless solution to the problem?
I would definitely invest in a good CPU fan and some thermal paste. Is this a socket 478 or LGA 775 P4? If you don't know, what is the model of your motherboard? Also, you may want to organize the cables inside your case. I bought some tubing from Radio Shack and grouped the cables together and my airflow now is very good. However, a CPU fan won't do much if you don't have other fans removing the hot air, so how many fans are in your PC?Newegg
has some good fans and thermal paste (Arctic Silver). I have the Thermaltake Blue Orb II, which I highly recommend. At 1600 rpm I can hear it, but it is the most quiet fan in my system and at over 70 cubic feet/minute, it blows a ton of air. Zalmann also has some very nice, quiet CPU fans.
If you really want a quiet PC, you may want to consider water cooling, but it will cost a lot more than air cooling.
While we are on the subject of cooling, are Ram Heatsinks worth the while? Or is it gimmicky?
I don't have any, and I don't think they are necessary in most situations if you have enough air moving around them.