I've been known to shoot handheld at 1/30th with the Sigma 10-20, it's an extremely forgiving lens. I also went form a D50 to a D200 and never looked back. There's nothing wrong particularly with the D50, but the D200 just does everything a little bit better and has superb ergonomics. The D300 does look incredibly yummy too.
Well isn't the rule of thumb for avoid camera shake 1/focal length.
That's what I use, I often end up shooting for a 1/10th with my 10-20.
And if you're even half-steady you can go further depending on your stance/hold.
Back to topic...
I too have a D50, but if I had the money (£600ish) I'd want a second hand D200. The main reasons for this upgrade would be:
-on body controls (no more menu pushing for metering, bracketing etc)
-better iq
-higher resolution
-better hold (I assume, don't think I've actually held one

)
-better noise handling (for gigs/indoors stuff)
-commander flash mode stuff (to trigger my SB600)
-metering with old manual lenses (I love my 'Cimko' 28mm f2.8, at £12 it was a bargain, but with a D50 metering is full manual. I'm pretty sure the D200 can meter for old lenses.)
-to look the business
I wouldn't go for the D80 though,
http://www.kenrockwell.com/nikon/d80/vs-d200.htm