It's hard to now exactly but it looks to me there is about 20 - 25% more headroom in the 5oox. They say they can put 5 '63 spring reverbs, a plate reverb, a V Tron in the 500X. At that point it won't even accept a volume pedal so it's pretty much maxed out.
I tried it on a 500 and found I could get the same result adding the volume pedal with 4 '63 spring reverbs. The estimation that the '63 spring reverbe uses about 20 - 25% DSP gets me to my conclusion.
I really think the 500X is more an update than an upgrade. It's a continuation of the 500 with a bit better specs, hence the name (it's still a 500). I don't think this is bad since it also sells at the same price as the 500, which to me is also a sign that it is essentially an update.It does mean L6 will be supporting the HD line for the time to come.
I also feel that there is a chance there could be updates on the 500X that won't go to the 500 because of the different patch-file extensions. This seems to point to a future incompatibility between them (they seem to be compatible at the moment).
It could be that the 500X will at some point get amps, effects or features the 500 won't get. Nobody knows the memory limit in these things. Maybe the memory of the 500 is now about filled up and can't get many more amps and effects. It could be that they also extend the memory of the 500X in order to be able to give us more HD amps.
Anyways these are just my speculations, nothing I got from the rumor mill.