I don't think I've ever gone the minify route with X/HTML.
It's ugly, harder to maintain and of limited benefit.
I don't know any details of your web site. I find a good fast server, compiling the programmed bits and good attention to code design can give lightning speed.
If it's just markup a good fast server still applies. (I've had ISP's move web sites to alternate shared machines to achieve this.)
Working on images is always a good idea and can make a difference. (Note: it's not always the same format that gives you best results, gif, png and jpeg all have their place in the sun.) I'd put that ahead of minification.
I know that some smart people set things up to compress the payload from the server. (gzip) There used to be some problems doing that but I believe it's now much better. I imagine that will also outperform minification.
For a discussion of this issue I found
http://stackoverflow.com/questions/728260/html-minification which gives additional hints, including tools that advise you on what to do.
Another thought. If your site has a lot of JavaScript writing it, be very careful. I've recently seen several high profile sites that do this and simply get it wrong. Their code takes so long to run that you just leave the site before it's done.