Until we know more it would be folly to upgrade, I would recommend using a different browser for the time being.

From the Sophos article the malware spread is Zeus which is Windows based.

Zeus? As in banking-details-stealing trojan horse Zeus?

As someone who has Opera installed on their home machine, which they also use for banking, how worried should I be?

Would Malwarebytes be enough to find and remove the trojan?

EDIT: http://malwaretips.com/blogs/zeus-trojan-virus/

Virus scan FTA: https://www.virustotal.com/en/file/8ecbca0de44c82d1c7ffced28...

That shows MalwareBytes detecting it as Trojan.Downloader.szb, and it does appear to be Zeus or a close variant. I'd be concerned.

> It is possible that a few thousand Windows users, who were using Opera between 01.00 and 01.36 UTC on June 19th, may automatically have received and installed the malicious software.

At least for me, Opera always asks for Administrator privileges before installing anything, so if you didn't happen to install updates in the timeframe given, you should be fine.