There's no reason that other x800 cards were not listed. Those are fine, too.
The 550 cards are not better, no. Different chipsets and software is involved.
Just remember that Windows XP is required. So a build specifically for capture is ideal. Get a good dual-core, quad-core, or even single-core, using AGP, PCIe or PCI ATI AIW cards. Save modern editing/web/etc for a new Win7+ build.