Is it possible to connect an AT power supply to the Amiga?
Yes, I did it many years ago but I don't have that info handy any more. Try a search on the net. Here is one link
Amiga/PC PSU If I recall correctly, you can also use a C128 power supply for the A500. The power rating is lower, but as long as you don't connect expansions like external floppies and such, it will do the job for a basic A500.
possible to connect the RGB exit to a PC monitor
Not directly. You would have to buy a "scandoubler" to connect an A500 directly to a PC monitor. As a quick solution attach the composite video output to a VCR, then the VCR output to a TV. The picture resolution is poor of course, but it will do until you find a cheap "flicker fixer" or scandoubler. Check here
Amiga Hardware and choose flicker fixers and doublers from the list to learn more about them and the different models. Then maybe you can find one for sale in a search.
And can I make a paralell / serial network with my Amiga <-> PC?
Try this utility from aminet
Twin 1.1. You will have to make your own cable, but it allows file transfer between PC and Amiga. It's not realy a "network" share, just file swapping.
And if your handy building your own projects, you might have a look at this....
IDE for A500Good Luck.
Plaz