Shoot that thing for a week and then pick up your K100d... It feels so small! The K20 was such a great upgrade for me... good luck, it takes a bit to get a feel for it but once you do, it really starts to shine.
Congrats... all those buttons will soon enough be your friend and you will wonder how you could even take pics stumbling thru the menu system on the old K100d...
But keep that K100d, I still use mine for club shots, and the ISO 1600 is still good... and for going places with sketchy surroundings...
I love the handling of the K20
but my little old K100 can still produce.
K100d - ISO 1600 - tamron 28-75