if you say that the temperature is 45 celsious, that is ok, it can go up to 55 and remain ok, that cpu works that way
about the 2x and 4x, on agp i never found deep diferences untill i played games, on normal tasks, there is no diference
before you changed the video card, did you cleaned agp slot?
the capacitors, this are the capacitors when are in good shape
when damaged or inflated or leaking fluids
it means that those are not working, between their multiple functions is regulate voltage to some components but when they fail, you can experience random failures like the ones you describe
those components can be changed but when one fail, the others will fail too, probably withing weeks after the first one fails so i normally recomend to change the mainboard
since this is a old amd mainboard, it is cheaper and better for you to replace mainboard, cpu and ram
you say you are not a techie, but don't worry, just ask and we will try to answer as best as we can
"The fan is noisy. So a new fan may solve the problem"
what fan, where is located, how much noisy? depending on the speed it spins, it determines if it is working or not
if it is not the cpu fan but it is the video card fan, that is a problem, if it is a case fan, in this machine, i would just unplug it, a duron overheating is uncommon but if you live in a hot area then replace it or we can explain how to clean it if you like
if it is the psu fan, better change the psu