No present computer, Mac or PC, has native decoding of H.264 in the video chip. All H.264 decoding is handled by the main processor, with DRAWING assistance by the video chip. No decoding assistance. This means that H.264 video plays exactly the same on a 2.0 GHz MacBook with GMA 950 graphics as it would on a 2.0 GHz iMac with an nVidia 7600GT (if you could do that processor/video chip combination,) as it would on a 2.0 GHz single-processor Mac Pro with an ATI X1900XT or a Quadro FX 4500. (Again, if you could get that configuration. Although, realistically, the faster front side bus of the Mac Pro would help a LITTLE.)
The reason you can edit HDV just fine, but not play H.264 is because HDV is based on MPEG-2, which isn't as demanding as H.264. The requirements (from Apple,) to play H.264 are as follows:
480p video (852x480, 24 fps): 1.25 GHz G4 or higher (I can confirm that these play fine on a 1.25 GHz eMac.)
720p video (1280x720, 24 or 30 fps): 1.8 GHz G5 or 1.83 GHz Intel Core Duo. I can say that a dual 1.42 GHz G4 barely squeaks by on this, and a 1.66 GHz Core Duo works fine, but a Core Solo is at a loss.
1080p video (1920x1080, 24 fps): Dual 2.0 Ghz G5 or Core Duo. This is firm. It does not play well on a 1.8 GHz dual G5, or a 1.83 GHz Core Duo. And any single-processor machine, don't even think about it.
If you can transcode the video, try putting it in MPEG-4 or MPEG-2. Both are less demanding to decode than H.264, but take up more space for equivalent quality.