Found a couple at ISO6400
ISO6400
Nice fish shot!
And to the OP--yes! Nikon or Canon! [Or Pentax. Or Sony].
How is the Sony A850, full-frame looking? Full-frame gives very high performance, even if the lens is "modest" in capabilities. Sony's now selling a full-frame and robust build quality body for below $2,000 now, and the Sony you currently have is, shall we say, a bit dated now. With a full-frame camera, you should be able to under-expose ISO 800 is RAW mode by 1 to 1.5 stops and recover later in post processing, thereby getting higher effective ISO settings AND the added benefit of higher shutter speeds or smaller apertures. This method of under-exposing in-camera at your camera's best High-ISO setting,and then making up the difference in post-processing usually yields better end results than shooting at uber-high ISO settings, where the in-camera noise from the signal gain at max ISO ruins the capture from the get-go.