|A USB tuner will use slightly more CPU time than an internal bus tuner. |
Your CPU is definitely fast enough to handle 480i, 480p, and 720p digital TV. The Athlon 64 3800+ is probably required for 1080i HDTV.
The problem with the Athlon XP is that it lacks the SSE2 instruction set. Many H.264 decoders use SSE2 and SSE3 to speed up HD video decoding (and deinterlacing, as in the case of 1080i HDTV) in software.
"they even say the PCI-E works with a Pentium II/3/4 or the old AMD K-6"
It will work with any board that has a PCI-E slot: late model P4 and Athlon 64 boards or newer. Since you'll be using the USB version, this is something you obviously don't have to worry about.
Dual core Opteron 185 @ 3.2GHz on A8N32-SLI Deluxe
4GB of CL2 PC3200
2x 8800GTS in SLI
Blu-Ray and HD-DVD
X-Fi Titanium for PCI-E
Samsung 24" LCD