That review Michal Jerz did ( http://my-symbian.com/other/preview_n900.php ) was pretty thorough. Good stuff and well work reading.
...The following screenshot shows output of "free" command in N900's X-Terminal, listing total, used and free RAM and swap (virtual memory) after a fresh restart. As the amount of free RAM (52 MB) probably doesn't look too impressive at first sight for a Symbian OS phone user, a word of explanation is needed here. Unlike on Symbian OS, where physical RAM is the only available operating memory, Linux (and so the N900) additionally uses swap, i.e. "virtual memory". In huge simplification, active tasks and processes are kept in RAM while inactive (or infrequently used) data are moved to swap. The N900 not only has additional 768 Megabytes of swap, but it is stored in a dedicated, fast NAND memory. That's why the size of free RAM is not even half as important as on Symbian OS, and when checking how much memory you have left for running additional applications you can safely SUM UP the remaining RAM *and* swap, just like on the screenshot below where total free RAM+swap is well over 800 MB. And in case of the N900 it'll always mean hundreds of megabytes, even after many hours or days of uptime and multiple tasks running. When you launch a lot (and I mean A LOT) of apps the machine may slow down a little bit (due to swap handling being slower than real RAM, and the system having to copy data back and forth between swap and RAM) just like e.g. Windows slows down when its pagefile is heavily used, but that's actually the only negative effect. Don't expect to see any "Out of memory" errors on this machine. I tried really hard to get one, and I ended up having over TWO DOZEN of applications running at once and multiple browser windows open, with some 30-40% performance drop being the only result and the system still letting me start new tasks and open new windows... so I just gave up....