On the software side you may find this interesting
http://www.mythtv.org/
Anyway
Make sure you have a motherboard with a bios wake up facility
http://www.malloc.de/tools/wakeup_clock.html
and you dont have to leave it on all the time.
You can get cards with built in hardware mpeg 2 encoders, so you can encode real time just like a proper recorder, and it doesnt slow anything down.
e.g.
http://www.scan.co.uk/Products/ProductInfo.asp?WebProductID=65225
You can also get Freeview cards and even satellite decoders, or even dual encoders so record 2 programs at the same time.
You can get pc cases which are very quiet, designed for AV purposes , that fit under the TV and can be controlled with a proper remote control (+ cordless keyboard and mouse).
Choose as big a hard drive as you want... how about two 400Gig hard drives.
Choose the DVD writer you want, multi format/ dual layer
(blue laser etc when it comes out.).. have 2 dvd drive...
watch any format you want, divx etc...
Share stuff over a network.
If you have a broadband connection you could even connect remotely (from work) to set up a recording. (use wake up on lan)
Electronic program guides (Digiguide etc)
And basically end up with an almost future proof system as you can upgrade bits as new stuff becomes available.
The main problem is probably the cost
. and I havn't seen any cases with a clock etc on the front..
Have fun
Steve