if you already have some good lenses with sony mount, why not invest in a new sony camera body? surely recent sony DSLRs can go higher than 400 ISO!
I actually don't know if your lenses are good, but if they don't suck it would be a bad idea to invest in a new camera system. As of now, Sony makes some pretty good DSLRs and you can get their excellent lenses from both Zeiss and Minolta.
You would save a lot of money, I wouldn't throw an AF 50/1.6 and other lenses away just because my current body goes only up to 400 iso. If you save on lenses you can get a high-end sony body, and you can always get other nice lenses later.
please note I never used Sony gear, and I'm not getting any money from them :-) I just think it's wiser, if you're not a pro, not to change everything you have because your body is old. Lenses never get old.
now my opinion on new stuff (nikon, as that's what I use and know, I just won't mention canon to be honest):
- there are beautiful macro lenses,
they sure don't come cheap but you
can stay quite under 1000 $. nikon calls these "micro".
- beautiful fast tele primes, they cost
a lot. those you could use for birdwatching.
- tele zooms up to 200mm or 300m, not
very fast, but very cheap and sharp.
- mid/high-end tele zooms, around 2000$
I don't know about other brands, but with nikon you really get different lenses depending on the camera, if it's DX(cropped sensor) or FX(full frame).
Most nikkor lenses are made for FX or 35mm film, they sometimes cost a lot, but you have a lot of choice (also used) and you can find excellent stuff at a good price/quality ratio. If your camera is DX (aps-c sensor) you also get some cheap, still optically amazing lenses. But you can't get, say, the 50mm f/1.8 or 50mm f/1.4, nikon's best designs; for normal purpose you need the 35mm on DX (a good lens anyway for 200 $), the 50mm you can use for portraits.
Problem is FX bodies are expensive professional stuff.
If you plan on buying a nikon body (which, anyway, I already advised you against) you can get some good macro lens from Tokina. It's, IMHO, the only brand actually as good as nikon at making lenses for nikon cameras, and they have better prices.
UPDATE: macro and (usable for)birdwatching nikon lenses at a decent price (all prices(rounded) at my local store, except where otherwise noted):
- Tokina AF 100mm f/2.8 Macro - 379€
(both canon and nikon mount)
- AF Micro-Nikkor 60mm f/2.8D - 400€
new on ebay (this is a beautiful high quality pro lens)
- 60mm f/2.8G ED AF-S Micro NIKKOR -
- 105mm f/2.8G AF-S VR Micro NIKKOR -
birdwatching (VR is optical stabilizer)
- AF-S VR 55-200 f/4-5.6G ED -
around 200€ new on ebay
- 70-300mm f/4-5.6G IF-ED AF-S VR
Nikkor - 590€
- Tokina 80-400 / 4,5 - 5,6 MM AF-D
(this is available for both nikon and
canon) - 630€
- 80-200mm f/2.8 D-ED AF Zoom Nikkor -
- a real birdwatching lens: 400mm f/2.8G
ED AF-S VR Nikkor - 9360€
super cheap, both birdwatching and macro if you want
- Tamron AF70-300mm F/4.0-5.6 Di LD
Macro 1:2 - 170€ (canon, nikon, pentax and sony mount)