My question is, why is Maya still so slow, with heavy geo in the scene?
Speed of geo is primarily about your graphics cards performance and whatever operations you do on top of the geometry. Not memory. Memory affords you to put more geometry in th pipe but it does not nessecerily speed up things (seriously if you can afford maya you can afford 8 gigs of memory end of story)
For example, I place 2 heavy oak trees in the scene, and moving the camera around
is as slow as it used to be back when I used to work with 32bit with only 1 GB of RAM.
Well if the hardware is otherwise same that about sounds right.
I thought Maya would use all the available Memory by default?
Only if it needs it. If your scene uses one gigabyte then the computer will use 1 gigabyte. IF on the other hand it uses 3 gigabytes then a system with gigabyte is in trouble. Tough seriously speaking no point in using a 64 bit syystem with less than 4 Gigs of ram, but better yet go to 8.
will 32bit 3rd party plug-ins work with my 64bit Maya?
No
Choosing hardware is not as easy as just tweaking one variable, its simply not at all easy to say what matters. Ive necountered situations where people threw literally 100,000 in hardware on a problem only to find that their bottleneck was their internet connectivity.