The performance differences between Ubuntu and Windows (32-bit, or 64-bit) in my opinion are rather null. I would tend to think that Ubuntu can give you better performance if you are a hard core linux guru and are building your own kernels, own optimized builds of blender, etc. But, judging by your knowledge of linux from your original post, you probably won’t notice a difference.
As far as RAM usage, it’s true that the 32 bit versions of Windows (XP, not sure about Vista) only allow a 3GB limit, while the 32 bit version of Linux only allows for 4GB. If you don’t know this already, the 4GB limit comes from the math 2^32 = 4GB.
I don’t know why 32bit XP has a 3GB limit, but you should also know that after XP is finished addressing hardware and such, applications can only address about 2 of the 3GB. No matter what operating system you are running a 32 bit build of blender on, it will always crash at around 2GB. There was a “large address capable” blender 2.44 build floating around here somewhere (I have it on my machine at home) that is capable of addressing 4GB without crashing, but you obviously need 4GB of RAM and a 64bit Operating system to use that.
I run Blender on Vista x64 with 4GB of RAM, and I can push blender pretty far with that.
Anyway, like I said, you probably won’t notice a difference across the operating systems. What really makes a difference is your models (how efficient you are with vertices), textures, materials, that sort of stuff.
Use what you want, and don’t think twice about it.
Also, what hardware are you running this on?