based on the test results i see, iso performance is definitely 2 notches up, not just 1.
but some things to remember:
1) build quality , i haven't touched a k-x before, but most people seem happy with it, so i'd take their word for it that this is a nonissue, since many of the people happy with it have used the k20d before.
2) weather sealing - absent from k-x, no biggie unless you like shooting in the rain, i know i don't.
3) no af confirm red light, so you cannot see selection of af point.. for k-x.. i haven't tried it yet, don't know if it's an issue.
4) no iso100, big minus for me... when you like low shutter speeds.
5) single wheel versus double wheel, to be honest i like single wheel better still.
6) aa versus lithium ion, age old debate
7) lower megapixel lor, but like anyone cares these days, unless it's below 8.
some other things, but these are the main differences. i think it's not very fair to compare k20d to k-x, since k-x is probably following on from k100d --> k100d super --> k200d --> k-m --> k-x, the fairer thing to compare the k20d to would be the k7, in which case the k20d still gets trounced significantly.. but hey, it's good enough for me for now.
actually I wanna ask about these 2 cameras because of the price point. brand new k20d is just around 100 bucks more than K-x. I wanna get K20D but seeing k-x showing more shines each day, I re-think my choice.