these for the last year or so, the 137B version. It's been great and it only has an i3 processor. It's responsive in terms of it's reaction the the remote control and I've never had an issue with stuttering or failed recordings. I also run xbmc on the same box so sometimes it's recording in the background via npvr while I'm using xbmc and again, no problems at all.
While more cpu always seems like a good idea one thing to keep in mind is that more cpu can equal more heat, which can mean more fan noise.
I have a 64gb SSD boot drive and a 750gb WD cavair black drive for recording. It's running win7 64 bit and 8gb memory. If I did it again I would only use 4gb of memory as 8gb seems to be overkill. It also takes a bit longer for the machine to come out of hibernation as it has to load 8gb back into ram. I'd also stick with a 32 bit OS, from other pc's I have, it seems more reliable on average. Just my impression tho.
For my xbmc library I use a synology nas. I think you could record direct to a nas from npvr if the nas is always up - I don't because mine isn't, hence the local WD recording drive. Synology aren't cheap but they are reliable and trouble free which is what I want in an entertainment system. They will also run sickbeard and couchpotato, two very convenient pieces of software. Worth googling if you don't know what they do.
fellow countryman here 4 channels? As in RTP1 + 2, SIC, TVI? If so, in my opinion: go digital (TDT/DVB-T). Find yourself one of those simple usb tuners (pen sized), and you're good to go. I had a Hauppauge Nova-T before it went belly up during a thunderstorm. I'm now using a cheap AF9015 based tuner (the $10 variety.) and it works alright.
My Celeron E1400, with 2Gb of DDR2 handles four simultaneous on those channels (sometimes with 2 overlapping recordings on the same channels - therefore more than 4 files being saved), plus two additional (analog) recordings from my Hauppauge PVR500 when I need to record the other 35 channels I receive through Cabovisao's analog service. IIRC my hdd is not even SATA2, so no real need to worry about hdd performance.
So I sometimes get 4 to 6 files being saved/recorded at the same time. No troubles. CPU is idle when it comes to recording (in my case), since my sources are either digital (the 4 channels) or hardware-encoded by the PVR500 (the rest).
So if your aim is to setup a recording system you can get away with a low-end setup. Heck... my current setup was an upgrade from a Pentium 4, 512Mb DDR setup, with the same tuners (plus an additional analogue tuner, since removed from it). My only aim when I upgraded was being able to play 1080p videos.
If you plan to use it to play your videos, anything than can play 1080p (an average graphic card that does hardware decoding will suffice) will do.
Of course, more is always better But remember the rule of thumb: more power = more heat, more heat=more noise from fans dissipating it.
I never found a Windows 7 driver for my AF9015. It may depend on the brand. Mine was a Twinhan.
AMD A75, 4 Gig, 1000 Gig
1 x kworld USB dual DVB-T (IT9135)
2 x Hauppauge Nova-t PCI