1) Higher memory consumption(5-10%)
2) More space on the hard drive for installed 32-bit libraries.
3) Memory is still 32-bit libraries.
The advantages of x86_64
1) In the binary distributions(such as Ubuntu), most applications will work going from i486 to i686. While there turned off most extensions of the processor, like sse and mmx. In juniper this is not essential since the Assembly comes with a i686, nevertheless the difference is still there.
If you are using x86_64 system, all packages will be compiled with compatibility with x86_64 that includes a far greater set of extensions, for example sse2, which means increased responsiveness and visible performance improvement.
2) No need to use brake crutches type pae.
3) In some operations, like encoding/decoding video/audio performance increase will be very substantial.
As you can see no much difference. Using х86_64 the performance win and lose in the memory. Using x32 — on the contrary. Given that memory you have 4GB — I would choose performance because it little never.
Tales of mythical glitches 64 bit systems — just stories. Flash a long time ago, completely normal(as it is possible to flash) works or through nspluginwrapper, or you can even use a native x64 beta version from Adobe-labs.
All sorts of blobs, like Skype and Google earth, are also working without any problem, though, and pull over a bunch of 32-bit garbage.