It would be very hard to get a total of 2GB of RAM on the new iMac. This is because it has 512MB soldered on the motherboard and one empty expansion slot. You currently have a 1GB memory module installed in that slot, and you can replace it with a 2GB module for a total of 2.5GB.
But do you really need 2.5GB?
The answer depends on the types of applications you're running and the size and number of documents you are opening. For most people 1.5GB is more than enough. But you can check the real-time memory demands on your system by running "Activity Monitor" (use Spotlight to find it) and looking at the free memory indicator. If this is low or if you find that your system is significantly slowly down when you open and work with your documents and applications, it might be necessary to increase memory.
Remember also that operating systems use the hard drive to temporarily store parts of main memory to free up space. This feature has several names including disk paging, swap file/space, and virtual memory. The concept is very simple: Allocate main memory in 'pages' of a certain size such as 16K blocks (page size varies) and keep track of when each page is used. Least-used pages can be swapped out to disk, making new room available. However, even this system can be overloaded if there are no more least-used pages to be swapped out. In this case, the operating system finds itself in a very tight spot and it must continuously swap pages of memory to and from disk just to keep everything running. When this happens, your hard drive runs forever, it seems. This is called 'thrashing' and it's a sure sign for a memory upgrade.