As for Firefox and Chrome I'm not sure, but Safari I know will not allow you to "down-grade" and it will not allow two version to co-exist. The same issue with Internet Explorer, that you can not have two versions co-existing nor does it allow down-grades. My recommendation for ultimate compatibility testing is to have Virtual PCs running DOS (text-based, single-threaded application support), 3.1 (16-bit browser support), Windows 98 (32-bit MS-DOS support) and Windows XP (NTFS support).
On DOS run any browser you can find that actually works, 3.1 can run Internet Explorer 3, Windows 98 can run Internet Explorer 5 or 6. Windows XP can run Internet Explorer 7, then you have Internet Explorer 8 installed outside of your virtual machine.
As for testing Safari, if you want to test the original you need an old PowerPC. Find yourself an ancient Macintosh on eBay. You can test Safari 4 and Safari 5 on Windows XP or outside of your virtual machine. Since Safari 5 re-wrote the core engine for HTML 5 support and a new DOM-heavy javascript model it would be good to test in both. Safari 5 broke a LOT of websites.
For testing Chrome I don't actually think you'll need multiple versions. Google never changed the core engine and the browser isn't very old. Any chrome is just as good at rendering pages as the next. The only difference is how it handles settings, history, cache, etc.
For testing Firefox, I can't really help you. I don't know much about how it works or the core engine, but it's probably good to assume that the large version numbers (version 1 to version 2, as opposed to version 2.1 to 2.2) indicate a large change in the core engine. Try to put Firefox 2 on the Windows XP Virtual PC and run Firefox 3 outside your virtual box.
I also recommend using a text-based browser like Lynx outside of your virtual box. Any blind people who browse the web depend on text-to-speech software, and text-to-speech software usually acts on a page like it shows up in Lynx, not as it shows up in the HTML markup. It's good practice to always make sure your website makes sense in a text-based browser like this.
Then, as always, make sure your website validates. http://validator.w3.org