I believe (for clarification) that you are referring to the behaviour whereby a page currently open in a tab will be "flushed" from memory, such that the tab and it's title remain but when you switch back to it will have to load the page again in order to show it to you.
This is new to Safari 5.1, and mirrors the similar actions that happen in iOS devices with mobile safari.
Unfortunately there is no way to stop this short of reverting to a previous version of Safari, or using a different browser. You may have plenty of memory, but with Lion Apple seem to have gone down the road of trying to make things as efficient as possible. To use a car analogy, just because you have loads of fuel in the tank, doesn't mean they can't change the engine mappings when you are idling at stoplights to save even more.
That's not to say that it achieves its aims all the times, as the page reloading may be more of an issue to you than the internal efficiency of the OS, but it's how they are doing it right now, and it's similar in other areas, such as programs that can be automatically closed if they are considered to be inactive. Some of it is bringing across some of the "anti bloat" methods over from iOS, some of it is change for changes sake.
So, in summary, you can see why they might want to do this, but it doesn't quite work as seemlessly as it needs to for people not to care, but there's nothing you can do about it without you change your choice of browser.
I don't know if this is exactly what you want, but if you open Settings, then click on Safari, then scroll down and click on Advanced, then click on Website Data you will get a list of websites that have stored data. You can click Edit then remove the data from just the sites that you want to remove stored data from.
Best Answer
You could try the "Reset Safari" under Safari settings. Its right below Private Browsing (at least on my computer). you can uncheck everything but clear history. I tried it, and wasn't asked if i was sure I want to clear history. My source