That's a good question - and I can't really answer it. Also I'm not personally familiar with the *istD (in fact why am I even replying
)... On the Pentax K200D and K20D, the ISO range covered by auto ISO is adjustable, so I'd suggest that if yours only covers 200-400, you may be able to adjust it to cover 200-800 for instance if you so desire... I'm sure *istD owners will chime in on this aspect...
As to your question, the way my K20D works in Tv is that it will adjust the aperture until it's wide open before it starts to bump up ISO. I tend to use Av and M to the exclusion of all other modes because I like to choose the aperture specifically to suit my purpose.