Can you post your specs? I suspect that your computer might not be fast enough to play the 720p movies. Haven't tried using GOM player but if it really makes movies look extra gorgeous then I'm pretty sure there is some extra processing going on in the background. Have you tried coreavc? Coreavc uses the least cpu power in playing hd movies but also slightly tradeoff some quality. As for upgrading graphics card, only upgrade to the newer gen cards if you're playing hd movies in h.264 format or bluray/hddvd disk. If you're watching movies through x.264 format then there will be no hardware acceleration.
Core 2 Duo Intel 2.4GHz
2GB DDR2 5300 (667) RAM
500 GB HD
Nvidia 7600 GT 512MB, HDCP compliant
On-Board 7.1 Channel Audio with S/PDIF and Optical Out
Logitech Z-4 2.1 Ch. Speakers
22-inch Dell P1230 CRT set at 1600x1200, 100Hz
Vista Home Premium
Before I tried GOM player, I've been regularly using VLC player which handles those large hi-def files in mkv containers distinctively smoother with no audio synch problems. However, GOM player does produce better picture quality because of the high level of customizations available. Parang may kabigatan nga lang. Kapag 1080p rip na, lalong mabigat ang mundo nya.
For those using both VLC and Windows Media Player, notice how much more vibrant the color saturation of WMP is compared to that of VLC when playing standard avi stuff and DVD. Unfortunately, WMP cannot play hi-def files in mkv containers while VLC is rather almighty, albeit the color reproduction may seem to be rather dull. Since the contents are eye-popping hi-def in the first place, I didn't really mind the color saturation until I tried GOM player. And like I previously posted, it's ridiculously gorgeous (comparatively speaking). But because of the stutter and out-of synch audio issues, I just stuck with VLC.
Yeah my PC is likewise connected to the Xbox360 HD DVD add-on drive and it handles playback well enough using PowerDVD Ultra at its present configuration.