Hi Jos :)
Yes, there are some performance benefits but it really depends on the scenario. Many Linux applications have lived a lot longer in a 32bit environment hence they are better tested than their 64bit counterparts. As for the minimum RAM required it's the same as the 32bit version (see
http://www.debian.org/releases/jessie/amd64/ch03s04.html.en). However I would suggest 256MB minimum if you want to have a window manager installed. The thing with 64bit is that pointers require double the space compared to 32bit ones hence the increase in RAM requirements (this is most notable with Windows where you can see increase by factor 2-4 in this departement). A basic installation of Debian however (the minimum 128MB RAM is suggested for
terminal-only installations; I've tried running an LXDE-based Debian on 128MB and it wasn't that great of an experience) includes very little software that accordingly doesn't blow up the memory requirements noticably. The more you install and run, the more RAM you will need. You can balance things out a little bit by adding some extra swap if necessary. I have been using Jessie from its first day of final release with XFCE on it and the memory footpring of an idle running Debian system is really, really great.
Note also that on 64bit you can run/develop 32bit software using the
multiarch feature Debian (any many other Linux distros) has nowadays. Read
https://wiki.debian.org/Multiarch/HOWTOto see how to enable it. It is important to notice that doing so requires downloading the 32bit counterparts of all required libs which on systems with small storage space (HDD/SSD) would have an important impact.