n a nutshell, the D300 will make fantastic looking 20x30 inch prints at ISO1600. At ISO 3200, you'll be able to get great looking 11x17 inch prints.
With the D3, at ISO 1600 you'll be able to get fantastic looking 30x45 inch prints. At ISO3200 you'll be a ble to get great looking 20x30 inch prints.
So, ask yourself, just how large of a print do you plan on making on a regular basis? Keep in mind the costs for mounting and framing prints goes up exponentially once your frame dimension exceeds 16x24 inches. For example, a good quality 16x24 inch frame will cost between 50 and 100 dollars. Bump the size to 40x60 inches and your probably looking at a frame that costs well over 300 dollars. BTW, I am talking GOOD metal frames with anti glare glass, not the cheap plastic junk available at wallmart.
BTW, I have found that an 11x17 inch print properly matted in a 16x20 inch frame is just about perfect for in home display. It's a size that's small enough for you do devote a wall to your images but large enough to be visually interesting. I'll do an occasional 16x20 print in a 24x28 inch frame but any larger just is too dominant in most houses. Here's one tip, if you ceiling height is only 7.5 to 8 feet, you'll want to keep your frames down to about a 24 inch height. If you hang too large of a print on a wall with a low ceiling it just ends up looking like wallpaper instead of a framed peice of art. The simple fact is that it takes a BIG room to make a large print look like it belongs there.
So, here's the bottomline. If your just going to make 11x17, or 13x19 inch prints, the D300 will suit you just fine. It'll also work fine for prints up to 20x30 inches but the 30x40 inch matt and frame will limit where you can put a print that large, basically over your fireplace or perhaps over the sideboard in the dining room. And yes, I matt every print that I frame. Call me old fashioned but I think that mattless framed prints look tacky.
I should also point out that buying the D300 will leave you funds to buy some fast primes. Which will help balance out the somewhat better high ISO performance of the D3. For a museum, you'll probably want the 30mm f1.4 Sigma. For portraits, how about the 85mm f1.8? Want somewhat long and fast, try looking into the 135mm f2 DC Nikkor. Want great and long, try the 300mm f2.8VR. you get the idea, that extra 3000 dollars can help you ge dome lenses with some real drool appeal. I should also note that the last 3 are full frame, so you'll be ready when Nikon releases the D400 in 2 or 3 years in the FX format.
PS. I would evaluate the D3 as having about a one stop advantage over the D300 at high ISO values. However, the DX format has about a 1 stop wider Depth of Field compared to the FX format. What it means is that at f2.8 on the DX format you'll have the same DOF as you would at f4 on the FX format. So, if you shoot an image at f2.8 with the D300 at ISO1600 you'll get basically the same image as you would shooting at f4 and ISO3200 with the D3. Same DOF, same shutter speed, same exposure level. Since both camera have basically the same resolution it boils down to a tossup unless you plan on making some really large prints. The reason that the FX format has an advantage for larger prints is because the larger format can resolve more fine detail than the smaller format, even when the camera resolution is the same. That is because a lens will resolve a certain number of lines per millimeter on either format and the larger format has a sensor 1.5 time larger, so it can capture 1.5 more lines across the frame. However, since the fine detail where this is visible is so small, it takes a really large print in order to see the difference. Which is why your print size is what should drive your choice and nothing else.
PSS, please note that I didn't mention any use of these cameras for electronic display. The reason for that is simple, the best monitors out there are limited to about 4mp reslolution. Since you would be down-rezzing any web image, either of these cameras is overkill for web use. For web only use, the D2Hs would probably be a better choice.
