I've been messing around with HDTV PVR systems, so I thought I'd post some of my experiences for any who want to read. This board looks like it doesn't see too much traffic, though, so who knows.
With HD, of course, you have a few methods of delivery: OTA (over the air), Cable, and Satellite. Cable* and Sat require an adapter box (* more on this later), and over the air will require a good antenna.
In addition to the referenced HD wonder and MyHD cards, there are some others on the market now: Vbox makes some nice stuff which is available in PCI, PCI-express, and USB interfaces, a company called DViCO makes some similar products, MyHD has a newer model card, and ATI has updated their offerings.
The state of the included software is still pretty abysmal, especially if you're interested in viewing/interfacing with that software on a TV via remote control. Windows XP Media Center Edition 2005 supports OTA recording of HDTV via any of those cards, and several linux and mac solutions support similar configs. As long as you can get a decent antenna set up, OTA is an easy way to do HD PVR work. The guide info is authomatically downloaded, and the interface is as TIVO-like as it gets. The highest bitrate OTA HD is 19.2mbps, so storing to your ReadyNAS shouldn't be an issue in any network other than a wireless one. Similar configurations are possible on Macs (using elgato's USB equipment) and linux (with MythTV or FreeVO).
Satellite should probably be written off altogether for HD PVR work. There is a company called 169time that modifies and sells modified HD DirecTV receivers that interface over a USB connection. They include software that allows those devices to be programmed from a PC, and I think they even support MCE 2005 for that purpose. I've never seen one in action, so I can't speak to it. The prices on the modified boxes seem a bit steep, but HD DirecTV boxes are kind of expensive, so I guess that's where a good part of the price comes from.
If you're using digital cable, you can request a box with IEEE 1394 (Firewire), and the cable company is legally obligated to provide you with one. You can use a mac as verden is describing, or you can use a program called FireSTB to allow an XP MCE computer to schedule and record shows from certain cable boxes over the firewire connection. Again, the most I've seen as far as bitrate from those sources was concerned seemed to be about 30mbps, so you should be fine as far as network speed.
Early last year a system was put in place that allowed for a digital version of the old "cable-ready" TVs. Called QAM (although other transmission systems also use QAM coding), this allows newer digital/HDTVs to receive digital and HD signals over normal cable. Depending on the carrier, all the signals may be unencrypted or just the broadcast stations (your local NBC/CBS/ABC/etc) affiliates will be unencrypted. HDTVs with QAM often have firewire ports that can output to devices called AVHDs or AVDiscs. These are basically just hard drives with interface chips that allow the TVs to talk to them. Newer mitsu/Toshiba/Samsung etc. TVs have simple options for scheduling recordings from channel X at time Y to Z, which could then be played back from the AVHD device. There is a product called firebus that allows your firewire-equipped windows PC to operate as an AVHD, and data could be written over the network to the readyNAS.
Additionally, DViCO's Fusion 5 PCI/USB/PCIe products support QAM tuning, so they can record and schedule recordings. Unfortunately, you're limited to their interface software because no media center PC product (XP MCE, SageTV, BeyondTV, etc) supports tuning to those QAM frequencies. I've read that mediaportal does support this system, but the few posts I see about it indicates that CPU usage pegs out at 100% and the video stutters. I can't confirm this, though, so your mileage may vary. Allegedly, QAM will be added in Vista, but we'll have to wait and see. I use a system called Record This which was produced by one of the regulars on the AVS forum. It's a command line utility that parses Zap2it's XML channels data downloaded by a program called XMLTV. I set it to automatically run once a day at 2AM; if a show is found in that day's guide data, RecordThis just updates the file that DViCO's software refers to for scheduled recordings and records that file. The resulting file can easily be played back through any of the major media players.
Finally, there are cable card solutions on the near horizon that were announced by ATI. Cable card is, in essence, a standardized way of authorizing digital channels for TVs without the use of a cable box. The card itself looks like a PCMCIA/PC Card, and plugs into TVs that support the cablecard system. The card tells the TV what channels you have and haven't paid for, and you can then tune into those channels. Vista will support cable card, but only thorugh "authorized OEM products." It's unclear if this means you'll just need the ATI cable card adapter (let's hope) or if you'll need to buy a pre-built system with all the DRM restirctions to keep you from uploading shows (more likely). Since MCE works fine with ReadyNAS products, you shouldn't have any trouble using a folder on the ReadyNAS to hold your recordings.