Best Video Card for non 3d/compositing work in Premiere CS4?

Question

Hi all, I'm looking to optimize my system where I can to speed up the editing process. I edit 1080p footage from the Canon 5D MKII (h.264 MOV files)using half-res h.264 proxies, but CS4 seems to hang a lot in the timeline view and clips just disappear and instead show green. I'm switching to half-res MPEG-2 (.m2v) proxies now and so far that problem seems to have gone away (still testing), but when timeline display quality is at "High Quality" (vs. automatic) there's still some chop during slow pans (not seen with "Automatic" or "Draft Quality").
Anyway, I'm looking for the weakest point in my system and suspect the video card. My system is an 2.66 i7 processor, 6GB 1333 ram, 1x160GB sata II HD (OS/Apps), 3x1TB SATA II RAID 0 (paging file, data), and an $80 ATI 4650 512 MB video card running Vista 64.
So all that said, what's the best general graphics card for editing HD video in Premiere CS4? I don't care about 3d or compositing at this stage, just basic editing of unrendered (timeline playback) HD video, maybe sometimes 1-3 HD layers, some basic color correction effects (b+w, contrast, etc), dissolves, text overlay, some clip speed manipulation.
Many seem to recommend the Nvidia quadro cards, but I haven't found any information to suggest they help with anything other than 3D/compositing, which I don't need (and therefore don't need to pay the premium).
There's also the Nvidia CX card, but it's main feature is touted as encoding h.264, whereas I could care less about that and ideally need decoding of h.264 in the timeline to be better. I haven't seen any great reviews on this card of heard of average user testimonials, probably because it's $2k and it mainly marketed as "zoom more fluidly in photoshop, export h.264 4x faster", which seem like minor issues vs. "better realtime playback of HD video".
I sort of rambled, but if any informed CS4 users can help answer the question regarding the best video card for general CS4 editing use (not 3d/compositing stuff) I'd love to get more insight.

Answer 1

So in Media Encoder CS4, I tried encoding one of the clips that plays back the least smooth in the Premiere timeline (a slow pan across intricate patterns on a sand dune in death valley) to the following:
Format: P2 Movie, Preset: DVCPRO HD 720P 30
(this was the only format in media encoder that mentions DVCPRO HD)
I noticed when trying to customize the setting, that there is no 30 fps option, just 23.976, 29.97, and 59.94. I just went with 29.97 as that was the closest to the 30 fps of the source MK II footage. The width and height were also unajustable at 960x720 for the DVCPRO HD format.
Playing the output in the timeline view of Premiere was comparable in smoothness to the m2v file. I looked at my resource monitor during playback looking for bottlenecks, and the CPU does seem to spike at 100% initially when playing this clip back. I reverted to the m2v version and also saw the same CPU spike. I also reverted back to the down-res'd mp4 version, same spike. Moving display quality down to draft does allow for smooth playback here.
So I guess there's something about this clip that is CPU intensive to play back in the timeline in any of these formats. The original 1080p version plays back perfectly in both quicktime and VLC player.
Anyway, for now I think I'm content with using the m2v proxies as a big chunk of the clips are fairly smooth, and using a reduced quality display mode for clips that don't play as smoothly is accpeptable for now. At least so far I'm not seeing the hanging/video not displaying (and instead showing a green screen) that I was with the mp4 proxies.
If anyone has any other suggestions as to how to get smoother timeline playback for these proxies (or the original h.264 .MOV files), however, whether it be a hardware upgrade (e.g. better video card) or a different proxy codec, I'd love to hear them and give it a shot.