TS-410 & TS-419P are good choices for your home NAS setup.
TS-410 is already powerful enough to stream up to 7 HD concurrently.
If you want to perform more tasks at the same time and higher performance, then TS-419P is the better choice as it has 1.2GHz CPU & 512MB RAM.
PS3 normally cannot playback MKV. MKV needs to be transcoded.
If you need to stream via UPnP, here are the supported formats:
• .ASF - Windows Media Video (codecs: WMV video and WMA audio)
• .BMP - Bitmap Image
• .FLV, .F1V, .F4V - Standard Quality Video (codecs: Flash Video: Sorenson Spark H.263 video codec & MP3 audio codec)
• .GIF - Graphics Interchange Format
• .JPG, .JPEG, .JPE - Joint Photographic Experts Group image
• .MP3 - MPEG Audio Layer 3 (codec: MPEG Audio)
• .MP4 - High Quality Video (codecs: MP4: H.264 video & HE-AAC audio)
• .MP4 - MPEG-4 Video (codecs: Microsoft MPEG-4 v1, Microsoft MPEG-4 v2, Microsoft MPEG-4 v3 & ISO MPEG-4 v1
• .MPEG, .MPG, .MLV - MPEG-1 (codecs: MPEG-1 video & MP2 audio)
• .PNG - Portable Network Graphics
• .WAV - Linear Audio (codecs: Uncompressed PCM-encoded & ADPCM-encoded)
You can also use other media players such as your WD Media Player or QNAP NMP-1000 to playback MKV files via Samba or NFS connections to NAS.