Sharks stuff will play anything, but not to the fullest. You should install it though, as it doesn't interfere with ideal playback if it is an option. If you are running 64 bit, don't forget Shark's x64 Components, and once installed, be sure to choose x64 playback (it chooses x86 by default). Also, Shark's pack is a safety net for random files you may come across.
Native Windows 7 playback uses Windows Media Foundation in favor of traditional Directshow, and in my case runs damn near 100% on the GPU. 1080p video/5.1 Audio registers about 7% CPU usage, and my CPU is average at best, it's a Core2Duo from 2006 running as 1x64 Bit. My Nvidia Geforce GTX 250 does all the work under WMF.
If you download DivX for Windows (free), it installs the WMF components for MKVs. Unfortunately I can't find an WMF transform for OGG files, so if your MKVs have OGG audio it won't matter. You'll be stuck with Haali, so you better have the proper old-school codecs installed, but so far OGG is the only thing that won't play in MMF for me.
One Ogg workaround I have found is 'Popcorn Audio Converter,' you can find it at hack7mc.com. It lets you batch process the audio streams in your MKVs. Just set up the guidelines you want it to follow (If Ogg then convert to AC3, if AAC convert to DTS, etc.) and the it automatically extracts the A/V streams, converts the audio, remuxes the original video with the new audio into a new MKV, and places it where ever you specify. It's pretty fool proof and gives a decent amount of options as far as quality, channels, etc.
Once the Ogg is gone in favor of AAC, MP3, DTS, AC3... All playback will be handled with hardware and WMF, and you can actually use your machine while watching HD video (2+ displays of course).
Trust me, you would rather be using WMF if you can.
All the best.