Sometimes we roll out new features in our control panel without announcing them. We usually do this so that we see a trickle of new users initially which helps identify any bugs or missing functionality before the entire userbase starts hammering the new features. :)
Selecting your VPS kernel bit-edness (32bit or 64bit) is one feature that's been available for a wee bit, but we haven't made an effort to spread the word on that. You can find this option inside your normal control panel in the 'vps info' box. Or you could go there directlly here.
You'll find a list of the kernels available to you with some comments by our kernel maintainer alongside most of those. If you're on a 32bit distro (most users fall into this category), it's fine to enable a 64bit kernel; you're not going to break your setup. But before you go running out and switching over to the 64bit version of your favorite kernel, make sure that you really need all that bit-ed goodness.
Your memory usage is going to go up once you switch over to a 64bit kernel. A lot of that has to do with integer size and memory pointers, both of which are now twice the size. You're not going to see a doubling of the memory required, but you will see a bit of a jump. If you're already running tight on RAM, you may need to increase your RAM after you changeover.
What about server performance? Will that be any faster? Most likely no. All of the speed comparisons I've seen show only slight performance differences between them. Those test sometimes favored the 32bit version and sometimes the 64bit version - far from conclusive evidence.
Well if there isn't much of a difference, why are we having this conversation then? Per-process memory availability is a big one. With 32bit kernels, you're limited in the amount of RAM that a single process can access. That limit is 2GB which is plenty for most VPS users, but if you're running large memcached daemons or just want to have a huge jvm stack allocated to Tomcat then you may want to consider going to a 64bit kernel.
In additional to your choice of kernels, we do offer native 64bit distros for a fresh install. You can do this from your control panel if you'd like to replace your entire VPS with a fresh image with the associated 64bit libraries.