I'd say you'd be best off with one of the two top Pentax flashes. I have both, and very much like both. At first I was hesitant about the FGZ 360, as it's small size and lack of swivel head seemed like real cons; so I first purchased the FGZ 540. I still have the 540 and use it frequently, but if the two, the 360 is the one I carry in my walk around bag. (actually I have two 360's) If I really need a swivel like effect, I just take the flash off and control it wirelessly with the K10D's built in unit (in command mode the on camera flash won't affect the outcome of your photos).
For me the wireless abilities, and the fact that I know for sure they'll work with my camera and other flashes is huge for me. If you do still think you need more power and a swivel head, then go for the 540, but I personally find the 360 to be a real workhorse. My 540 rarely makes it out beyond use as a slave on a light stand with an umbrella attached to it. Even then, my other 360 compliments it as a near equal in that role (I use two flashes on stands wirelessly with one 360 on my camera to control them for large group shots). For the price you're talking, plus the lighter and more compact convenience of use, I'd say you couldn't loose with a 360.