Why do I get ORA-27102 ON WINDOWS 64 BIT if I increase the memory to 2.5GB
How much real memory free does Windows task manager show, before you start up Oracle?  If less than 2.5 GB, that's the problem.  You can try increasing memory_max_target slowly from its current value, say in 256 MB increments, until it errors again, then I'd back it off by maybe 512 MB to give some headroom.