The phone has 208MB of RAM, with ~64MB or so reserved for the phone and PDA functions, which leaves ~140-145MB or so available for the actual running of programs (which includes windows). I generally start with ~80MB free, running down to ~70 over the course of the day (idle), using Titanium as my UI:
Just for comparison, the HTC Imagio ships with 288MB of RAM, with ~170 available for running programs (Including Windows). Most users report about 60-40MB free with TouchFLO running. I was able to get my Imagio up to about 70MB by turning off TouchFLO and disabling every HTC service. My experience has been that although the Omnia 2 has less RAM, it manages the memory much better than the Imagio.