The rom (software) of the Desire HD has already been ported to the older sibling the HTC Desire. I must say, the performance is incredible! This means that you could potentially buy a regular Desire for cheaper with similar results, if you are low on money that is. A reason for why the HD rom is better than the old Desire rom is that the HD has the newest version of sense (HTC's user interface), which has been packed with new apps and other tweaks. One cool thing is that you can now change the theme of the phone. The map app has also been upgraded to allow for offline browsing, which is in my opinion one of the best changes.
Here is a video of a Desire running the new HD rom:
