I'll let other posters go thru all the hookup options on the 921, or you can look around for some of the manuals, etc. I think the biggest problem you might have is that I've read you can't watch HD and SD at the same time on the 921. ** Edit - I see you added that you want to have both HD and SD working at the same time. Just keep your 501, it's only an extra $5 per month for the "extra receiver" fee.
However, you'll LOVE HD on your plasma. I've had a NEC 50 for about two years, and the HD programming is gorgeous. I currently have a 501 and 6000. It's almost painful to watch NTSC (Never Twice the Same Color) broadcasts, and it's almost painful to not be able to PVR the HD stuff.
Yes, it's true, that some of it is upconvert, or in the case of ESPN-HD, just stretched "regular" pictures. It's also true that there are some HD channels that E* doesn't have. It's also true that some of the (particularly older) movies really show film defects.
Even with all that, you won't be able to believe how great the picture is. If you get a decent sound system, it truly is "just like being there".
I have a 921 on order thru satelliteguys.us, and I consider it a great addition to my entertainment setup.