I also believe bad things will happen (device crash) if a swap partition is unmounted whereas it's OK to unmount swap files on the fly. The internal card will be automatically unmounted when the battery cover is opened so you increase the likelihood of your device crashing by using a swap partition unless you never intend to open your battery cover while the device is powered up!

I also think the size of the swap file created by the memory applet is limited by the size of available memory - the N800 has 128MB RAM and the 770 has 64MB RAM and the latter could only create a 64MB swap file. In the past users have attempted to manually create swap files larger than physical RAM but this tended to result in instability for some unknown reason.

As Karel says, search the forum...