Be careful with terminology, folks.

o RAM: RAM is always physical RAM. In the N900 there's 256MB of it.
o There's no such thing as 'virtual RAM'.
o Swap space: Resides in the built-in flash storage (there's 768MB of it on the N900), or could be set up on microSD as well).
o Virtual memory: This is (on Linux) the sum of swap and RAM. On the N900 it's 1GB (768+256).

Virtual memory is what the applications can potentially use (not considering some limitations added by the kernel). Virtual memory is the only thing applications know about. If you have no swap space allocated, only the 256MB physical RAM, then your virtual memory size would be 256MB. There's no such thing as 'not having virtual memory set up'. You mean _swap_ here.

o Swap space is always slower than RAM. Maybe in the future this could change, when/if some of the stuff being worked on in the labs manifests itself. Then maybe we'll have RAM, SD cards, USB sticks, other mass storage, all built from the same magical, non-volatile, super-fast stuff.

o Adding swap will not make for a faster device, only for a device which can sustain more applications running at the same time, or using more data, without going down in flames. There was a myth, for a while, that the N800 and N810 would be faster if you enabled swap space. This was only in the imagination - I tested this. And there's no rational reason it should be faster, except in very carefully thought out scenarios that won't happen with normal use.

o Adding excessive amounts of swap _will_ make your device slower, eventually. Particularly if you keep your applications (e.g. browser) running at all times.