Both Intel and AMD are introducing new ranges. This will mean slightly different motherboards needed. Also PCI express seems to be the way forward for graphics cards (That and a power station bigger enough to run a small country) click here
Gigabit networking is becoming increasingly common 10/100/1000
A quick punt around thgis site should help with the latest news. click here
Either jump on the pc bandwagon now, or wait a few months for the new technology to kick (6 months for it to come down in price)
Hard to say at the moment, but you can guarantee that what ever you buy there will be something bigger and badder around the corner.