If you want something that's not going to be out of date anytime soon, but is pretty affordable... I'd go with a Core i7 920 processor and load your OS on an 80gb Intel X25-M SSD. Buy a large Seagate 7200rpm drive for your applications.
I'd go with a Gigabyte motherboard. You should be about as set as you can possibly be at this current moment in time.
I currently render all of my HD content using this same machine configuration, less the SSD. The difference it's made for my productivity is huge.