I built a quad core 2.4ghz with 4gb ram to run avid media composer at home. The os is windows xp pro (hence the 4gb limit) but people are seeing real performance improvements with vista business and 8gb ram. If you go down the avid road, budget for an nvidia graphics card (quadro1800 minimum) as avid now takes advantage of 'distributed rendering' which takes the load off the cpu and shares tasking with the graphics card. stick with the quad core as a minimum.
You could probably build this for 1k sterling, but a great resource for diy-ers is the videoguys website, where they have recipes to suit a wide range of budgets.
