I'm new to Android too... I assume that music can be stored on the mSD card, and videos too, right?
Absolutely.
Is the HD2 with the 16gig card a class 2? I have a 16gig class 6 that I'd definitely use instead of that, but having another 16gig backup would be nice
The Hero has 512 onboard memory for app storage. Theres a myth that theres a "70 app limit" of things you can install but its off-base. There IS an upper limit on how many apps you can install but its a matter of how large the apps are and not the number of apps (so if all your apps are really small you can have 100+ and if the are all very big then you might only have below 50). Around 70, with the average app size, tends to be where the threshold is reached and its a true headache.
I have about 80 installed and try to stay right under the cap. Apps that I find useful but rarely use I keep backed up to my SD card to access when the occasion arises where I want it, an example would be "Speed Test" which will measure your current download/upload speeds. Useful, but rarely used.
Basically what happens when you install too many apps and reboot your phone, you will get ZERO network connectivity, both voice and data. As the phone loads, all those apps you have installed are competing for memory and since too many are installed it can't handle it. You will spend about 5 minutes going through all the force closing of every app imaginable as the phone tries to forrest gump its way to life. The fix for that is going into your file manager and uninstalling a few of the biggest apps you have (and also dont use), then reboot the phone and it will work again.
It's very nice that the EVO will have double the memory of the Hero but I without native SD support for app installation, the issue will remain because its an android issue. So I will be able to have about 160 apps instead of 80 all things equal, which should be more than enough even for a power user like me. I still don't like that this issue is there, though. I'm rooted and could do SD card app installation but am currently too lazy to deal with the headache of teaching myself to partition the SD card like you have to in order to do it.