Hi,
first of all, there is no browser without memory problems, but Safari seems to be one of the worst. It makes definitly no diffference if you use 5.05 ore 5.1 concernig the memory problem. But there are seriuos additional problems with Safari 5.1 and the 10.6.8 update, so I would avoid generally the MacOS 10.6.8 update.
You can try things, like throwing off Adblock and keep off websites with many flash-ads or websites with not segmented table-displays. But all this will perhaps diminish the problem, not solve it.
My solution is to use Google Chrome instead, but only without flash, because shockwave is enormously processor wasting, much more than the flashplayer in Safari.
Opera always brings my processor to 140%
Firefox may be a possibility too.