As far as SATA... I have an Xbox original with a 1TB hard disk in it (which also run linux and MythFrontend)... i used this:$4.20 @ http://www.dealextreme.com/p/sata-hdd-t ... card-57841
Cheapo SATA--->IDE adaptor, no reason it couldnt be used in older desktops as well (had to hook it up to a desktop to lock it as part of the Xbox procedure)
Also, MythTV uses USB external drives REALLY well, my case only has room for 1 internal, there is a WD green drive there, then there are several WD drives in external enclosures mounted for use by MythTV.
And as far as horsepower goes, it really depends on how your recording... SD or HD... not much needed for a frontend(700MHz Xbox original works for SD). Our main Myth box is a frontend/master backend recording from STB's in SD through a PVR-150 and a PVR-500(3 tuners total)...
system.board.product = 'M3N78 PRO'
info.product = 'AMD Athlon(tm) 64 X2 Dual Core Processor 5600+' [2.9GHz 65W]
info.product = 'Future Technology Devices International Crystalfontz CFA632-USB LCD'
info.product = 'Hauppauge WinTV PVR-150'
info.product = 'WinTV PVR 500 (unit #1)'
info.product = 'WDC WD10EADS-00L' [S.M.A.R.T. Power_On_Hours: 20240 (2.31 years)]
In use 24/7 since: Oct 29, 2008
The main thing that will make a huge difference in what machine to use is what capture card you go with... hardware or software encoding, and whether you are recording in SD or HD.
Here is the usage from uptime i just got... load average: 1.01, 0.98, 0.80
currently recording SD on one tuner with a commercial flag job running... we have occasionally noticed some lag in the remote when the system is recoring 3 shows, flagging one, while watching a recorded show... rarely is all that going on (usually monday nights around 8pm
) that it's not an issue.