It's not an all-or-nothing scenario to have SATA and PCI.
My main ATI capturing system is based on an older AMD Athlon single-core, and the motherboard has SATA-I and multiple PCI slots. There were SATA-II single core motherboards, SATA-II with dual core CPU, etc. I've often found
Geeks.com to be useful for older hardware. While I do shop at Fry's or Microcenter, as well as
Amazon.com and
Newegg.com, most of my boards come from
Geeks.com, as I rarely build a new whizbang computer. My "new" computers are almost always of previous generations, because the hardware and software in use is not compatible with the latest hardware or OS.
New 6/8-core AMD systems may not have Windows XP drivers -- especially for the new motherboards. I'd avoid that.
Get a capture system based on modest specs. Leave the high-end latest/greatest for an encoding or editing computer (non-capturing).
For example: Right now, Geeks.com has a "Dell OptiPlex GX620 Pentium D 945 3.4GHz 2GB 160GB DVD±RW XP Professional Mini-Tower" for $191 shipped. It has anything you'd need for a non-AGP capture computer, aside from the capture card. Or, as an alternative, search for "AGP" and it'll bring up a list of boards. Find a board you like, then search for the CPU and RAM. You can build a capture-only box for under $200.
In fact, next month, we'll be selling some "capture kits" on this site. Computer motherboard/CPU/RAM + capture card + optional TBC. You'll have to only supply your own computer case ($25-50, whatever you want to spend) and copy of Windows XP. These were working systems pulled for various non-technical reasons. I expect they'll be gone within a week of being posted.