The answer depends on the type of applications running on those machines. There is no way to really tell until you run benchmark tests on a loaded server.
Taking advantage of memory overcommit, you can allocate more than 128 GB of ram to your virtual machines. You also need to consider ESX memory sharing where the ESX server will compare memory pages and free up additional memory by sharing the same memory pages between multiple machines. The actual difference between the number of idle XP machines with 1 GB of ram compared to 2 GB of ram won’t be that much if any.