I guess I would need to ask what exactly are you talking about??? and please take no offence here.
While I don't own a Bigma, I have the sigma 70-200 F2.8 and use it a lot with a 2x TC (making it a 140-400 F5.6)
I also use a 300mm F4 plus 1.7x AF TC (500mm F6.7)
YOur Bigma is in the same order or perhaps 1/2 stop faster (it is bigger in diameter after all) than my combos.
I have shot at anywhere upto 1600 ISO on the K10D with good results,
I have also posted shots with flash to freeze the image as opposed to using natural light when it is just too slow, and shots where I hand held the 500mm combo at 1/100th and 1600 ISO in the shade to get shots.
I am surprised you don't feel there is enough light to use your bigma. Are you really sure you are trying hard enough?
I have thought about the K20 as well, but decided right or wrong, that it did not offer enough above my present cameras (k10D and *istD) to have a go at it.
Last week, while just out for a walk, I stopped into a camera store and they were talking about the new (?) K30D and ISO 128,000.
I have to laugh about that, but maybe not. I think I'll wait and see. If the next pentax (K30D seems the logical number) has far superior high ISO, I will probably get one and get a Katz Eye for my K10D and use it for manual focus only. I will always keep the *istD because even though it is only 6MP, it does such a great job with true TTL flash.