Need advice on grahic card or something else

I bought a 24 inches monitor. It is very impressive to watch my HD footage on a real HD monitor 1920X1080. However I still have problem to watch my HD clips ( MOV h.264 and mpeg 2 ) smoothly, not choppy. So I bought a new Grahic card : Sapphine Radeon HD4670 Ultimate. The problem is not solve, only faster rendering. ( At least a improvement! ) . My CPU is Intel Core 2 Duo E6300.
Strange is I play my clip MOV format on QuickTime Player, still choppy..

CPU is Intel Core 2 Duo E6300
Sapphine Radeon HD4670 Ultimate
Ram 4 Gb

Anyone has an idea of how to improve or what is missing in my system.

Thanks for your suggestions in advance.

Pat

Problem could be fragmented or slow speed hard drive. Try to defragment first and check if it helps. Or use a separate HD for clips you previewing.

Edited:
I use the Performance Test program to check my hardware: http://www.passmark.com/products/pt.htm

I had a totally new HD and I found out the QT player play on 20fps instead of 30fps. (Thanks for Jake´s answer on other subject.) And it seems QT player did not use GPU power? Is it truth ? Is it a problem of weak CPU or weak GPU ?
I watch internet video from Apple site, the video are beautifully smooth with my old notebook. Why not on my PC with :

CPU is Intel Core 2 Duo E6300
Sapphine Radeon HD4670 Ultimate
Ram 4 Gb.

Pat

Pat,

I have a quad core 3.2 GHz processor with 16 GB of RAM and a very fast 750 GB main drive in my computer and I still get choppiness playing full screen MOV files in Windows. I think it's QuickTime and not your computer. I don't think it does enough caching before it starts to play. When I play them in VLC the choppiness goes away.

dnavarrojr,
Thanks for your comment.
It made me feel better after I heard that you higher end PC has the same problem too.
The new graphic card does a faster job when rendering. However is it quick to render a 14 minutes (or 4 GB long) clips into Time-lapse around 30 minutes time.
It seems my consumer software--Magix did a bad job for extreme Time- Lapse, like 14 minutes long moving cloud into 10-15 second. It becomes un-usable noisy and the black-grey-white are in steps. Not a smooth gradation as the cloud should be like.
Is a professional software, AE or CS4 can solve this problem. I guess so, stupid question??
Pat

HDD speed is rarely the cause for choppy playbacks of HDV or H.264 compressed clips. I guess it´s mostly the processor that isn´t fast enough to undo the compression "on the fly " for smooth playback.

Nevertheless, VLC Player plays almost all my HDV (m2t) files perfectly on a 5 year old P4, 3GB RAM. Much better than QTP or Streamclip and without washed out colors. I use Streamclip only for encoding now.

BTW: A good codec for previewing clips is DVCPRO, quality 100%. It is frame accurate like mjpeg but has DV bitrate, 28 mbps, and pretty decent quality at full HD size. Those files play like charm, and if they do not it´s very likely the clip that has a problem, not your hardware.

Stefan