I was interested in installing the SPDY mod for apache2, in order to improve the speed / latency of my website.
My major question is : does all the websites on my dedicated server will have to switch to SSL, or will SPDY be enabled only for the SSL enabled website ?
I hope it'll not mess up all my production website.
No, not all the websites on your dedicated server will have to switch to SSL. For SSL requests, if SPDY is negotiated via the TLS NPN extension, then SPDY will be enabled.