Thank you for your responses. My conclusion is that there is no particular justification for any Opera version beyond 12. I use the foxy browser (always the latest release) for financial activities. As for Windows, I have XP SP3, but have found no pressing reason to install it.
I am really attached to Opera voice feature so 12.02 is staying. I have XP SP2 + opera 12.02 due to upgrade issues. There is a creeping need for XP SP3 with non-Firefox browsers which is worth being aware of the issue. A lot of sites are being urged to change to the newer SSL certificate signing using SHA-256 hashing (currently SHA1 hashing is widely used). For example (https://www.dropbox.com) has done this, so any Chrome/Internet Explorer browser will fail unless they run XP SP3 or later Windows operating systems. It does not apply to Firefox as they use open source SSL library that supports SHA-256. You can test this by trying (https://www.dropbox.com) in each browser. But what about Opera 12.x and higher - so far I have not found Opera 12.x to have SSL issues it seems to avoid this issue and must therefore be validating the certificate hashing without using Microsoft library code. I think if the site supports both SHA1 and SHA-256 in the certificate it will work. An example of this is also (https://www.dropbox.com) it works in Opera. You may consider applying SP3 for the odd troubleshooting occasion when you need to test with working Internet Explorer or Chrome browser, although Opera fits this role a lot of important sites just wont display well in Opera 12.x and that will get worse over time. A few other Windows components are going to have signing problems on XP SP2 as cited here (http://bit.ly/18SGayU), so if they were important to your usage then SP3 may be a more compelling for your machine. If Opera and Firefox were not around running a workable browser on XP SP2 would be extremely difficult. It will probably be hardware that ends your XP machine before software or needing SP3