Optimizing Linux with cheap flash drives

Well, if you are stuck with 4 GB only (and presumably 32-bit), the there might be some mileage in putting swap on USB keys - but may I advise you to RAID-1 them: (and use different root controllers).

Incidentally, yes, 64-bit is worth having: it allows lots more RAM, it allows apps to *map* more than 4GB, and the architecture, though wasting some space on larger pointers, has more registers, so it's still a win. It also lets firefox address 8GB of swap (this is purely for memory leaks, so it doesn't matter that it's slow: fx merely has to write the data successfully, it will never read it again).

As for caching files, you *might* be able to do it with mplayer.