I recently upgraded to CS5, fully 64-bit, but because a lot of my plugins and etc are still 32-bit, I installed the 32-bit support so I could have premiere & after effects CS4 and CS5 on the same system. But this seems to have created a but with media encoder, When I do a render que in media encoder, it doesn't start at all, just hangs there saying contacting dynamic link manager. The only way to render out of Premiere CS5 is to use the export feature within it. I also get a problem with whenever I say export media, it says there is an update for the media encoder, when there is one for CS4 media encoder not CS5 media encoder. And all the processmanagers and mediaimporter are running in 32-bit for some reason.
I recommend getting the suite. Mixing and matching CS5 and CS4 not a good idea. I took the plunge and so far I don't regret it even though I have run into some bugs that are slowly getting fixed. CS4 engine not the same as CS5.