Since you're going to be CPU bound anyway, you can take advantage of the opportunity to get a low voltage, fanless GPU. This would be desirable for a living room PC. I'm not sure if the GPU's you mention can run fanless, but if you can find something decent that can that's what I'd get. According to a review I read recently, I believe the GF4 MX does accelerate MPEG-2 video decoding.
I have a P2-450, and am CPU bound with a Geforce2 MX. Nice thing about it is it runs fine with just a heatsink, never had a hint of anything close to overheating and it's probably as fast as anything the system can make use of. I've noticed that a DX7 game runs the same speed no matter what video mode I'm in, so that's a sign the GPU is on cruise control. I play DVD's full screen on this computer as well, it runs fine with no CPU load or stuttering issues. I haven't seen benchmarks, but I would guess that the GF4 MX 440 you mention would saturate the P3-450 CPU. I could be wrong though.
Some of the early AGP motherboards from the early P3 era weren't able to handle the amperage of some of the heavier video cards at the time. A friend of mine had his Abit P3-700 motherboard die 3 times (each time replaced under warranty). I'm convinced that it was because of his heavy video card. At the time, some motherboard makers didn't realize how rapidly the electrical load of video cards was increasing, and they cut corners on the AGP electrical specs. Due to heat, noise, and possible electrical concerns, its better to get a lightweight modern GPU rather than an old hotrod GPU.
AGP 4X cards are generally compatible with AGP 2X and 1X systems. I don't know about AGP 8X. The reason I say "generally" is because they may advertise that the card requires some particular AGP revision, and an older board might not have it. But it still has a good chance of working anyway. That's what happened to me with the GF2MX.
