AMD 64x2 4850e CPU (low power consumption), 780G based motherboard, sorry, I forget the exact model, but here's one of them
AMD's New 780G Chipset Has Powerful GPU : AMD's New 780G Chipset - Review Tom's Hardware, 2gb RAM, Technotrend TT1500C, Antec Fusion case, DVBviewer Pro.
It also has a silent 430w PSU, and I believe I went for a Silent Knight II CPU cooler. The whole thing cost me around £350 at the time, but I didn't get all the bits from the cheapest place, so I suspect I could get it closer to £300. In fact, I'm probably going to prove that, as I just gave my box to a friend, so I'm building a new one myself some time over the next couple of weeks.
Depending on which DVB software you go for, they usually have very detailed setup instructions at their forums, and if you're looking for info on softcam installation, the DVBN forums probably isn't a bad place to start.