Yeah, is the high ISO
really that much of a deal breaker for you?
yes and no. the FL problem is actually the bigger one for me. when the light leaves i can put the camera away and life generally goes on. the FL problem has been affecting how i see the world. 75mm isn't long enough to separate the details from the distractions but also not wide enough to match my natural FOV. It was fun and interesting to work around for the first year and a half (obviously i've done a lot of stepping back into plants, walls, etc.), but now it's just tiring and no longer fun. (before: "wooo!" now: "jesus christ i'm not that poor")
i'd like to have clean/usable iso up to 800-1600 (to match the d40 i shouldn't have given to a family member, but ok), but i'm not delusional enough to think anything beyond that is required for my shitty photos. part of this is sample specific since i have a very early d80 which has the worst of the amp noise issues.
Not enough light? Find some. Place your subjects near windows or other sources of light. Still no good? Bring your own light. That SB-800 you have should be great for most of your needs. Use it with the combination of your on camera commander to make for some interesting two light setups. (Personally I don't bother with Nikon or Canon's IR wireless lighting system anymore. I just go all manual with radio triggers or optical triggers. The SB-800 is awesome since it has a built in highly sensitive optical trigger. Just set it for SU-4 mode.)
i played around with the sb800 quite a bit (to the annoyance of everyone around me) when i first got it. i've been drifting towards PJ-style natural light and landscapes recently so it's been sitting since then. i happen to have gotten it for ridiculously little, so it's not that big a deal. someday i figure i'll put together enough courage to work with subjects that take direction, i guess.
I have the Sigma 30mm 1.4 that you mentioned and I use it all the time on my D40. It gives me great shots. Granted, I used to hate it since it was always front focusing. I had to send it and my D40 to Simga for recalibration. Once it was fixed, though, beautiful.
i went as far as buying one and when properly focused i liked it at big apertures, but at the time i refused to send it in for service on principle, so it went back. i haven't tried again because i don't think the extra stop is worth it for me if i have to exchange it for the corners at all apertures.
ps, for those who don't know, used 5ds are down to 1000-1300$ range used.