Both T-Mobile HD2 and HTC HD2 hava 576 MB physical RAM but HTC HD2 was reduced by software to 512 MB only. Radio v2.07 and above can unlock it to use 576 MB. Why do you need to install custom radio ROM that supports 576 MB? NAND Android and Windows Phone 7 need it. You need to have correct radio ROM in order to use NAND Android and Windows Phone 7. Just follow step by step video guide to install it.
In order to install custom radio ROM, you need to have HSPL installed on your HTC HD2. Follow this guide ~ How to Install HSPL4 on HTC HD2? to install. I used the latest radio ROM 126.96.36.199 in the video guide. You can use it or any other HTC HD2 Radio ROM version that support 576 MB.