There is no cost Helldiver. It is still the free Streaman that you mentioned. What Roger has done is make it usable in DX10 Preview mode utilizing this piece of payware software which converts planes.
DX10, which stands for DirectX 10, is a graphics program which helps the graphics card interact with your software application, such as FSX. Formely, with windows XP, DirectX 9 was the buzz. Now with Vista, DirectX 10 allows more features to be implemented in games with less impact on performance. Whether or not we have met that goal is subjective at best.
DX10 uses a more streamlined architecture to provide higher graphics with less of a performance loss overall. For example, shadows and whatnot take less a hit with DX10. There are many limitations however, such as the operating system (Vista).
Games have to be made to work with DX10 in order to achieve this level of performance, otherwise they will perform the same as they did in DX9. When Microsoft released SP2/Acceleration, they implemented some features of DX10 in the form of "DX10 Preview." It only gives a little less overhead and is not fully fleshed out. Some people however swear by it.
If you are interested in a rather small and to some subjective visual show and tell of DX10, watch this video Helldiver. You may make up your own mind.
http://www.youtube.com/watch?v=ouJ3pYhu43c
If you can, watch it in high quality, which is an option just below the video itself.
Anyways, for many, like myself, DX10 for FSX is still just a pipe dream.
If you are still confused, I can try to better explain it to you Helldiver. It is all rather confusing to me at times for damn sure...