is it possible? are they 100% working in proper condition?
do i get 1.5GB in combined of 1gb and 512mb?
will it increase the performance of FPS or others thing? (don't refer to Crossfire, talking about Vram only)
Is 1.5 GB good enough for 1920x ..... resolution?
The more important thing you have to look at is the GPU performance. You will get 1.5GB of memory to work with, but the CrossFire setup will run at double the speed of the lower-end card, which I'm assuming from your lack of info is the 512MB one.
Given what you've got, I think you'll be able to run that resolution nicely.
Getting a 512mb will hinder the 1gig card. Since the workload is divided the 1gig will be restricted by the 512mb card. In my opinion get yourself a 1gig card at that resolution, 512mb will start to struggle with aa/af.