Hi. Recently I have been experiencing the same problem. When I play minecraftm its process (javaw.exe) uses anywhere from 1.5 million K to 2 million K in Memory. I have reinstalled minecraft and java and still no change. This is causing my PC fan to speed up and make it sound like it is taking off like a rocket ship. I have java jre7 and jre6 both installed.
For one make sure all drivers for your system are updated. Two there are some serious bugs in 1.5 that need to be fixed. These will be fixed in 1.5.1 which should come out either this week or the next!
Please help!