|Basically, you can't.|
The problem is that you are in a grey area. If you do go to a 64 bit you loose 1G ram. See the official Microsoft web pages for that proof. There is a lot of issue to even consider a 64 bit os. Not all systems will work and not all applications will work. Your system has to have support for it in all areas. And you'd have have all drivers.
The old and not too trusted way is to use PAE. If you are brave then try this.
From the MS pages.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.