If you're interested in silencing a HTPC and are planning to keep most of your files on an NAS device, you'd so just as well to use a suspended 2.5" laptop hard disk, IMHO. This would be inaudible (providing you select a suitable model) and would cost a fraction of the amount of an SSD whilst providing greater capacity. If money is no object by all means go for an SSD but in this straitened financial climate, the cheaper option may be better!
If you're worrying about power consumption, just use S3 suspend to ram. My PC draws less than 1W more when in S3 suspend than when switched off and resumes to desktop within seconds whenever you need it. Very straightforward and easy to set up.
The other cheaper option, of course, would simply be to replace the hard disk in your PS3 with a higher capacity model - 500GB laptop disks are now available. This wouldn't solve the MKV problem though.