The A300 basically uses the same sensor as the Nikon D200. Nikon purchases Sony sensors for their use and then tweak it and slap it into their camera. The big difference you'll see in the sensor b/w the Nikon and the Sony is the speed. The A300 bursts at 3 fps while the D200 bursts at 5 fps. The A300 isn't a bad offering considering the price and the flip view LCD on the back. Sony bought out Minolta awhile back to get into the DSLR market and uses most of their mount and technology (similar to what Samsung did by buying Pentax).
The biggest drawback with Sony is their lack of lenses and accessories. What you're saving now on the body will come back haunt you in the end w/paying more for those in the future than the equivalent or better from either Nikon or Canon.
For example:
Nikon flashes: SB-600 ($185) & SB-800 ($315)
Sony flash: HVL-F56AM ($310)
Nikon 70-200mm f/2.8 VR: $1625
Sony 70-200mm f/2.8: $1980
Also, here are some important differences between the two cameras:
ISO: A300 limited to full stops; D200 stops can be adjusted in 1/2 or 1/3 increments
Image stabilization: A300 built into the body; Nikon VR built into lenses (same w/Canon)
Max shutter speed: A300 up to 1/4000; D200 is one stop better at 1/8000 (important - depends on how often you shoot at faster speeds or outdoors)
EV (exposure composition): A300 to +/- 2 stops; D200 is +/- 5 stops
Continuous shooting: A300 3.0 fps, unlimited JPEG, 6 RAW, 3 RAW+JPEG images; D200 5 fps, 37 JPEG / 22 RAW frames (important for bursting)
Viewfinder: A300 isn't as high as the D200 (harder to use if you don't use autofocus)
Weight: A300 is lighter by about 50% vs. the D200
Seals & gaskets: A300 isn't weatherproof; D200 is
If you're looking in that price range, I'd take a look at the Nikon D60, which compares much closer to the specs of the A300 than the D200 used above. (I used the D200 above since they use the same sensor, plus used D200 is hitting close to the same price range as a new A300). If you can squeeze a bit more for a camera, then I'd also take a look at the Canon XSi.