Allow me to intrude for a moment. I'm no expert here, but since I've wasted my life by drooling over cameras, here's my opinion:
...I intend to do a lot of night photography...
Since you're going to do night photography, get an SLR with :
- Good High Iso performace. I see most SLR have good ISO performance up to ISO 800, but only a few have good performance up to ISO 1600. I prefer a noisy ISO 1600 than a noise-free but detail-free ISO 1600 picture.
- Effective dark frame NR, which means after a 30sec exposure, the final picture is quite clean, without too many 'hot pixels'. And...
get a fast lens, preferably the f/2.8 variant. Bigger is better, usually. My 50 f/1.8 cost me $80. Of course the quality is not that good at 1.8, but stopped down to 2.8, it is good
...but for a serious amateur, with small and not so steady hands...
Well, I've got big and shaky hands. I want my IS, but I'm still trying to save up for that. So, I go out and buy a tripod. A cheap one, I might add. I've got mine for $20, and I must say that it is quite satisfactory. But if you like outdoor night photography, perhaps you'd better consider the bigger manfrotttos. They're quite heavy, but they resist wind nicely.
You're right about your considering of Alpha-100 and K-100. They've SteadyShot and ShakeReduction (don't you just love these names, which basically means the same thing? ), which means they've IS (or VR, your call) built in into every lens you have. I know I've been reading the K10D reviews too many times recently. Gotta love K-10's SR!
Finally, you have to decide what kind SLR you want to accompany you for life (well, I know it's a bit extreme), but once you invest in lenses, flashes, and all kind of things for your SLR (new eyecup, cable release, and so on), you will find it is hard to change into another brand, cause you'll find that all of your lenses, flashes, and accessories won't work on the new camera. (Well, okay, maybe the flash still work. But no TTL for you).
To close up this lengthy gibberish, here's my recommendations for you (just for reference, valid at the time of writing )
1. Rebel XTI body only (avg $700) + Tamron 28-75 f/2.8 XR DI (avg $320) --- sorry Canon ppl!!! --> XTI have good high ISO performance
2. Nikon D40 (avg $580) + ...
a. Tamron 28-75 f/2.8 XR DI (avg $320) or
b. Tamron 17-50 f/2.8 XR DI II (avg $370) - quite good for landscape - or
c. Sigma 24-70mm f/2.8 EX Aspherical DF (avg $ 400).
--> D40 have good high ISO performance
3. Pentax K100D (avg $580) (I'm not familiar with Pentax' own lenses, sorry, but Tamron make 28-75 for Pentax)
--> K100D have SR
4. And finally, how about Oly E-500? (avg $650 for the dual lens offer)
--> E-500 is a good deal