Dell would be a good choice for #1. As stated above, make sure he gets one with an upgradable MB for his graphics etc.
Per #2, I would agree with Phenious. I would look for a nice MAC to set him up with. To toss him into the PC world would probably throw him for a loop. If he is comfertable with the MACs platform then a new G5 would be a nice upgrade and would also keep him happy. The last thing you want to do is convince the guy to place a large amount of cash on a computer and have him unhappy. The G5's are very nice and do very well with both video and music editing.