My pages are W3C Valid, I cant test all browsers if my pages are show as expected but in IE and FF its shows my pages properly. Since my pages are W3C Valid, is it a guarantee that it will show fine also to other browsers?
Writing valid code will certainly help, but it's still no guarantee that your pages will work in every browser, on every OS. You might want to have a look at http://browsershots.org/ - it's only static and won't help with drop down menus etc.. but it's a start.
Kerosene pretty much summed it up. Valid coding can help towards cross-browser compatibility, but you should still check for inconsistencies in other popular browsers, especially in older versions of Internet Explorer. A lot of large organisations are dropping support for IE6 though, so we can only hope the market share will drop significantly in the near future.
If it works in recent versions of IE and FireFox, chances are you're good on most other browsers people will be using (Chrome, Safari, Opera mainly).
You definitely should install all major browsers, at least latest Firefox, Opera, Safari, Google Chrome and IE8. And also IETester for older version of Internet Explorer. Those two (IE6 and 7) are always causing most troubles...
Valid markup means you wrote the syntax correctly. You dotted all the i's and crossed all the t's. It does not mean you used it properly or, as others have said, the browser will display it as you wish due to quirks, bugs or inconsistencies.
The reason it's not a guarantee that it'll work on any browser is that W3C validates your code to HTML standards. Some browsers use different html scripts, therefore not all browsers will display the same HTML correctly. (i.e. IE cannot display some things that FireFox can such as vertical alignment.
I agree, browsershots is a great tool, it shows you what your pages looks like in all browsers(like 20 different ones). plus w3c for html and css, which helps when trying to makes the page complaint to a specific browser. websiteoptimization.com works great too. this reference shows page load times, loaded objects, and more. not so much for validating but to does have great information on your site