Marin, if you would please, just install the 64 bit host and check out the result.
It should take you no more than 30 minutes.
It is not a destructive change. It does nothing to your project.
Your UAD powered plugs will not be affected.

When running X1 in 32 bit mode I can adress 4 GB, so there is 1 GB headroom..

No, because Windows is using some of that memory too. 

Maybe I´m mistaken, I thought that running W7 64 with X1 in 32 bit mode I can use 4 GB for the Application..

Kalle, yes I´m just opening my taskmager while Sonar is running and check the ram usage

Hey Peregrine, maybe I should do it, I just remember at my friends studio, that whenh running UAD bridged, it causes troubles, like they won´t remember the parameters and such..

I would have ditched the UAD but I can´t work without my EMT 140 and 250 reverbs..

You can download and install meminfo - it's freeware - and you will have a realtime status of memory usage in your
notification area. When your machine boots, memory is allocated for the video card and the motherboard BIOS. Then
memory is allocated for the OS and then everything else that's configured to run at startup. On this 64 bit machine,
my current usage is at 2.95gb after startup. If I start 32 bit SONAR and open a new project, usage goes up to 3.17gb.
IE is running at about .062gb and there are only a couple other, very small, applications I could terminate if necessary.

Any 32 bit process in a 64 bit system will get a virtual 4gb memory allocation.  As you can see in my case, sitting
here at idle throttle, I'm already using 3.2 gb . Once you start processing, and add plugs, effects, temp files, etc.,
you run out of room very quickly.

If you insist on remaining in a 32 bit environment, you can try setting program priority higher for SONAR and
any of the applications you want to use. I'm not sure it will help because the system processes will just trump
your settings and take whatever they need. If you have a 64 bit OS, I think there is a 4gb switch you can set.
You can do a search for this, or someone else may have more specifics. The program has to be large address
aware, I think, and I'm not sure if this is true for SONAR - you'd have to check the headers. Even so, I don't
know if this approach would be of any help.

Thanks Peregrine,

I will install Sonar 64 these days and test it for sure..

Man, 3.2 GB idling status, that´s alot, I`ll get 1.68 GB!!