There's no hard and fast rule on this. Supporting IE6 and IE7 takes an investment of time and knowledge that you may not have available, but on the other hand, if you want your site to look as you intend, it's an investment that has to be made. So the question becomes: which is more important to you?
You say the "if I check the statistics of the pages, i noticed that almost half of the visitors uses this kind of browsers," which says to me that unless you're OK with half your visitors seeing something other than the design/layout you intended, you're going to need to make that investment or get the help of someone who can.
If that's not an option, you could try using some of the CSS "frameworks," like Blueprint or Grid960, and see if that's easier, but that will require a little bit of learning as well.
The other options are either going with a simpler design likely to work across browsers, removing the stylesheet for IE6/7 and letting viewers see the raw HTML document structure, or using table-based layouts if you know how to wield them (and contrary to what some people will tell you, there's nothing at all wrong with this route if it's the one that best fits the requirements of your project combined with the constraints on your abilities and resources).
