Hvae an old pc with intel 820810E onboard graphics and a pci matrox millenium g200 pci (8mb i think). Think the second is the better one but still get poor youtube videos even with latest driver. Am tempted to get one with more mb but how much is needed? Have seen some pci ones with 32mb and 64mb but do i need that much or should the ones I already have be up to the job?
You should have no problem doing low quality video on youtube, though HD will be a problem with either the onboard or the old g200. My old g550 wouldnt even do HD playback and it was much faster than the g200.
Make sure you have at least 1GB of system memory, and any modern graphics card made in the last 3-5 years (amount of vRam is not as important as how new the processor is). Also for HD playback you will want a Pentium D or newer processor on the motherboard, and preferably a Core 2 Duo or better. I hate to say it, but you may need to upgrade more than just the GPU.
Lastly stay away from Matrox cards. They are wonderful for business use where you need a lot of high resolution monitors to do spreadsheets and documents, but they are simply not made to do any sort of video, games, or 3D anything. Stick with either nVidia or AMD for your home GPU.