Memory remapping will push all that addressing above 4 GB, by making the system see the full 4 GB memory and share address pointers redirecting addresses that were previously overlapped. This does happen at a performance cost as well. On 64bit systems the same is apparent but of course with much more memory (about 8 Peta Bytes).
MTRR can manage this well and there is nothing from the how-to above extra you need do. One thing to point out is that some proprietary drivers such as ATI fglrx drivers do not always work with this feature. I have had them working before but at reduced performance because of the remapping feature, other versions just fail to find the graphics memory address so if your having X driver problems Blank screen, Lockouts, hard lock crashes etc then Disable the remapping in the bios.