1. AMD processors are regarded as better value for money, especially for gamers.
2. It all depends on what needs upgrading at the time. But in general, CPU, RAM, and video card upgrades are perhaps the most important.
3. Quote from my CPU guide
"It¬ís not very easy to compare the performance of Intel and AMD processors. An Intel processor running at 3 GHz is not equivalent to an Athlon processor running at 3 GHz. This is because AMD processors can run at a slower clock speed and yet match the performance of faster-clocked Intel processors.
As a result, AMD has named its CPUs on an equivalent Intel rating, not based on their actual clock speed MHz value. This helps to compare AMD and Intel processors. For example, an AMD Athlon XP 2100+ runs at 1.73 GHz, yet it matches the performance of a 2.1GHz Pentium 4."
Hope it helps!