I've only been using Opera for about 10 minutes now, and it's using an INSANE amount of memory.
Take a look at this screenie: 
That was only with 2 tabs open! Now with one tab open, it's using somewhere in the 90 megabyte range of memory.
Not to minimize your concern, but I'm not sure I'd call that about of RAM usage insane. Right now, in 10 Beta, I have two tabs open and it's using 84MB of RAM. You might want to try asking on the Opera forums at my.opera.com. It's always possible for Beta software to have memory leaks, and if so you'll want to report it there. Good luck!
I've been using Opera as my main browser now for several months and while I also find it uses a load of memory, it still is faster than Firefox. Right now, it's using 90mb. When one realizes that today's computers are shipping with a minimum of 2 gig of ram (many configurations include 6-8 gig of ram), what we used to think of as a memory hogging program is no longer the case. The programmers are probably writing these browsers with a minimum 2 gig installation of ram in mind.
My Firefox browser uses slightly less memory, yet it runs as if it were carrying a heavy weight on its back. Once a few add-ons are installed, Firefox 3 takes a bit longer to load, and with no script and Ad-Block, it feels very sluggish.
I tested Opera, and it is indeed very fast! I still like FF though, just too used to it.
True, Opera is still the fastest browser, but I mean compared to the memory usage of the latest stable version of Opera, the latest Opera 10 build uses a lot of memory. Not that I'll be switching my browser from Opera to anything else anytime soon!
EDIT: That memory usage spike must've been just a random spike. Opera is now normally using about 30-40 megabytes of memory.