Realistically you'll probably find better options if you go with a 32-bit PCI card. It will slow down the clock speed of whatever bus you plug it in to, so check if there's a slot available on a bus where you wouldn't mind it having that effect. (*)
There are some 8400GS 512MB PCI cards made by Sparkle, maybe other brands. I have one of them and it does support H.264 acceleration but probably not in the web browser. When I tried it I saw acceleration working in linux with mplayer. I presume it would also be accelerated in MPC-BE under Windows but I haven't tried that. It probably is not accelerated in VLC Player and I don't think it will accelerate flash video directly in the web browser.
Due to the limitation of data rate on a 32-bit 33MHz PCI interface, you really will want H.264 acceleration if you want to watch HD video on these cards. If the CPU decompresses a 1080p video stream and tries to send it across the PCI bus, it will choke. 1080p is roughly 2Mpixels * 24bpp * 30fps = 180MB/sec, way too much for the interface to support. It needs to send the compressed data across and let the GPU decompress it instead.
The other option, and maybe what you had in mind, is to get a PCIX card that can transfer fast enough to let the CPU handle the decoding, but those cards are so unusual you might have to compromise on finding anything that's competent with games.
NVidia did something really irritating and allowed 2 different GPUs to be used in so-called "8400GS" cards. It isn't merely a die revision, they have different features.
The older GPU from the original 8400GS is supposed to be faster in games but doesn't have usable H.264 acceleration. The later GPU used on later 8400GS cards has much better H.264 support and is what's used on the 512MB PCI Sparkle cards. From what I've read, I think that buying a 512MB card may ensure that you're getting the later GPU, while smaller RAM sizes could have the earlier GPU. If you want HD video playback then you won't want the older GPU.
8400GS is the lowest card on the NVidia side to have usable H.264 acceleration. There are later generation cards also which will be faster in games but they get more expensive. There's a 9400GT/9500GT, Geforce 210, GT430, and some GT520 and GT610 cards. The GT430 should be the fastest. I believe all of these cards are keyed to work in a 3.3v slot.
Not sure what's available from ATI with H.264.
* = It's possible some of these late PCI cards might support running at 66MHz, but that's just speculation. I've wondered if my 8400GS would do that but haven't had the opportunity to find out.
