Originally posted by Duarte Should I :
-> buy the K20D body..
OR
-> wait it out a bit and get the Nikon D300
None of the applications you mention require top-class AF or high fps so I don't see what the allure of the D300 for you is.
On the contrary, the in-body image stabilisation of the K20D should suit you very well and keep your lens expenditures down. The availability of some very good, fast glass for the K-mount also speaks in favour of the K20D (unless money is not a concern in which case the options in Canikon land are plenty).
As for high ISO capability, the recent K-7 review on dpreview shows that the K20D (sic) is right up their with the best when it comes to RAW quality. If you get yourself a decent noise reduction software like Neatimage or Noise Ninja you should be good and -- as opposed to Canikon -- can actually control how much detail you want to throw away with the noise.
Where the D300 has the edge is in dynamic range, which is perhaps ~1.5 stops better (not sure, check DxOMark results and make sure you use the "Print" tab for comparisons to compare
image rather than
pixel noise).
I'm pretty sure that Pentax/Samsung will eventually come up with a full frame body. You may have to be patient but sooner or later the cost of FF sensors will have come done and Pentax (hopefully) will have acquired the financial backup to do some FF lens development again.