I'm using a TerraTec Cinergy 1200 DVB-C, a single receiver PCI card. It has been working flawless in my htpc with ubuntu (currently 11.11).
There are some PCIe boards on the market, but I haven't found one with linux support (which is a shame).
I tried some USB receivers for DVB-T/boxer, but switched to PCI. The antenna cables had a tendency to pull out the usb module from the htpc.
Using tvheadend as backend and xbmc-pvr as frontend.