I went through the exact same process. The K-7 was calling my name after purchasing a K-x.
I went ahead and purchased the K-7 with the battery grip. I was really burning through batteries on the K-x, even though I was using rechargeables. The body does feel much better in my hands. The autofocus is much better. High ISO is still worlds better on the k-x. But I rarely go above 800.
I still use both cameras. I find that I am more likely to take the k-x with me when portability and weight is an issue. For serious photo shoots I always use the K-7.
I find that they compliment each other nicely, just depends on what I plan on photographing on any given day. Enjoy your cams!