I have lots of MP4 MKV MOV files of various lengths, resolutions, audio formats, audio and image quality.What should I convert them to to have most playback performance without quality loss?

[My research](http://pastebin.com/HXYrxQHw) ((>hour of) browser history)

To avoid HDD bottleneck I use a RAMDisk but some files are too big.

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PC Specs

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1. Dell OptiPlex GX280, newest BIOS
2. Intel Pentium 4 2.8GHz Northwood (32-bit)
3. 2.5 GB RAM
4. Hard Drives
	* ST3200014A
	* 2× WDC WD400BB-75FRA0
5. OS
	* Lubuntu 14.10 (kept uptodate, to be replaced)
	* Windows 7 Ultimate (kept uptodate)