I've been thinking about this for the past few days and had a few questions/suggestions.
Why exactly are you dissatisfied with the high ISO performance? Is it just pixel peeping that's getting to you or are you actually make prints that have an unacceptable amount of noise? If the noise is that bad, are you doing something to fix them up in post or are you amplifying that noise by upping exposure in post? I've found I can do pretty well with higher ISO on my K10D if I make sure to NOT underexpose and use some good noise reduction in raw processing. I don't use the Pentax software. I use
RAWTherapee . It's free and fairly similar to Lightroom (minus the file management part).
Another thought: IS will buy you faster hand held speeds, but that does not translate into subject stopping speed. So even if you can shoot at 50mm at 1/20s, your subject will be blurry unless it's holding still. Though you do have the 50mm 1.7 for those occasions. If your lens it racked out to 200mm you could probably hand hold at ~1/60. Then again, you might prefer some subject stopping 1/250 shutter speed instead.
So faster glass versus IS is a bit of a wash. IS applies to all your cheap lenses and will obviously be there in the future if you get faster glass. Faster (higher quality) lenses offer smoother bokeh and improved image quality/sharpness/less chromatic aberration.
Good luck with the decision!