Online help is your friend! Try here for video information including processor requirements:
http://onlinehelp.microsoft.com/en-us/windowshomeserver2011/using-digital-media.aspx
Maximum RAM is 8GB. You do not need any video card to stream video to clients. G620 should do all you need and much more.
Phil P.S. If you find my comment helpful or if it answers your question, please mark it as such.