As we all know that Microsoft and Mozilla had released the latest browsers IE9 and Firefox 4 recently, and I want to use them in our projects/sites, but I don't know what's the risk if I upgrade them, which means I'm not sure our sites can be shown correct or not on these latest browsers, if someone can give me some advices on this issue will be helpful, thanks.
When you say that you "want to use them in [your] projects/sites", am I correct in believing that you want your sites to test well against them?
If so, then I humbly submit that's the wrong way to look at it. The greater danger is in NOT supporting the new browsers, because your visitors will be using them, regardless of your decision. Therefore, you MUST test your sites against them, because if your site breaks, then your visitors won't return. (If these are completely internal visitors, like for an intranet, then you block your staff from being able to stay current with other current web applications.)
The real question is whether you want to keep supporting older browsers. For now, that's required, too. You must build your sites in such a way that they work correctly in ALL relevant browsers. IE7 is still used by a lot of people, so you might continue supporting it (for now). IE8 is a must, as is Firefox 3, Chrome 10.0, and Safari. (I don't know the current Safari.)
Many websites publish statistics on the commonly used browsers. It's your decision how low of a percentage you want to support. Personally, I would support any browser having market share above 10%, unless you're a VERY highly trafficked site, in which case, I MIGHT support any browser above 2%.
If you're concerned about how you can continue testing against older browsers, then Internet Explorer has a "Developer Tools (F12)" feature that allows you to change your rendering to match older versions. I don't know about other browsers, but I think you can have both Firefox 3 & Firefox 4 installed on the same machine. Until you have a way to continue testing Firefox 3, do not install Firefox 4, at least until it surpasses Firefox 3 in use.