I'm considering buying Carrara Pro 6 (I had an opportunity to demo it and I find it a lot more user-friendly than Vue).  My question is this:  Is C6 Pro compatible with Windows 64-bit version? If so, is it limited to using 2gb of RAM like most 32-bit processes?  Thanks!

I don't have any problems running Vista 64bit and C6 Std.(dunno about Pro, but should be the same). If I read forum posts correctly, a 64bit OS lets a 32bit app(like C6) run up to approx 3-4g ram limit. Hope this helps.

I'm running on XP x64 & it runs fine, that said yes, it is still limited to 2G since the core of the program has not been updated to x64.

Just to correct/add to my above post;... from what I read(correct if wrong), if the 32 bit app is "large address aware", then it should be able to use up to 4gb ram in a 64bit OS. In a 32 bit OS, by default, 32bit apps will only use 2gb max. 

Some programs are rumored to crash if they exceed the 2gb limits as well, so hopefully someone can confirm if Carrara is large address aware for those using more than 2gb and a 64bit OS.

I do have to correct you a bit.
Being "large address aware" allows using up to 3gb and works even in 32bit OS.
C6 is not "large address aware" and generally becomes unstable approaching 1.6gb usage. As far as I can tell some garbage collection is started around that level of usage and it never finishes. In very complex renders, using multiple threads, I've had memory allocation errors when getting near the 1.6gb usage.
Using a 64bit os and multiple core CPU's has an advantage when using your computer for multiple tasks but you should use some kind of small render farm (even 1 additional computer is a bonus) for all rendering to continue using your main computer even when rendering.

Thanks for the clarification jfbeute. I try to keep up with some tech info, but the net has so much to read(learn) lately, lol.

Im still new, but was curious to the crashing with some apps on systems at or around the 2gb limit(just a guess). Some are saying that Carrara will just use the disk swapfile when it runs out of physical memory, but it seems by reading the 1.6gb+ usage in any 32bit app seems to cause instability(at least thats what I see/read).

 Im sure DAZ is working to get a 64bit version out as soon as possible, cause many 3dcg users, pro and hobby, are already working with machines/scenes that surpass the programs 32bit limits ram wise. Even if the HDD swapfile worked perfectly, IMO, it isn't an option when it comes to speed for rendering(it slows waaaaay down).

The story isn't that simple. Many applications don't actually need all the memory they allocate all the time. Swapping doesn't have to slow down any application. A very good example is Poser 7, it now supports a way of swapping texture maps, having in memory only those portions actually needed at the time. It hasn't slowed down rendering and has allowed much more complex scenes to be rendered. In Carrara 6 you can minimize the program and watch how much memory is actually used (generally less than half a gig) but other memory has been allocated (and isn't in use at the moment, maybe not even needed in the rest of the rendering). Having more memory available (and in a 64 bit OS the limit is very high) leads to more memory being allocated and that leads to swapping. With care any application can be made to run without problems in a 32 bit environment.

That said a properly working 64 bit OS would improve many things. Currently the 64 bit Vista isn't stable enough yet to provide a good platform for software developers. So any hope that all 3D programs will be converted soon is idle hope, too many users will stick to a 32 bit OS for the time being and maintaining two systems is too difficult and expensive. Even MicroSoft itself doesn't believe in its own OS at the moment, the flagship Office package is only available in a 32 bit offering.

I've got Carrara 6 installed on Vista 64 (Russian version, no less) with 4 gigs of RAM and it's been working fine.