It depends on your needs and wants. It's an expensive want but is it nessasrily a need?
True a 400D is an entry level camera and it is designed to work well in most conditions. Most cameras will struggle in an aquarium environment. Last time I was at an aquarium I was shooting at 6400-12800 F1.4-4 on full frame. But your depth of field struggles on FF. Thats whats nice about APC you get more DOF, low DOF is great in portraiture but not in all other shooting. You have to step FF cameras down further to get the same DOF. Great yes we can now do that but again 6400 and 12800 on FF have similar complaints as 3200 on the 400D so really you might be frustrated with the DOF with a FF camera and you will be spending double the amount for a fairly niche amount of shooting.
Here is an example at F4 (24-105mm) on FF
IMG_0651 by
TomScottPhoto, on Flickr
This is F2.8 (70-200mm) on APC shot at 1600 and cropped.
Snake by
TomScottPhoto, on Flickr
The DOF is not that different. As F2.8 is roughly F4.5 on APC.
I firmly believe FF is not for everyone and APC is great. The cost outlay for the camera and glass to go with FF is expensive and people seem to have this "if I don't have full frame I can't shoot in low light" and FF is a must! But it's not, it's just the next best thing.
The 5DMKIII is my first full frame camera, main reason I bought it was I make a living with it, print big and the agencies and clients want the best IQ. So the cost outlay isn't really an issue as it makes it's money pretty quickly in a few jobs.
Would I go back to APC now I have full frame? No not with current offerings. Why? Im very happy with every quality of the 5D it's fast, light, small, reliable, IQ kicks ass, print big, feels like it would survive an earthquake and it shoots very well under low light. So I answered most of your want with that answer. But upgrading to FF costs a lot add 50% to a 6D for the 5DMKIII and if I wasn't reliant and printing A3 or less APC kicks ass, you get deeper DOF lenses are longer, 70-200 is a 110-320mm add a 2x extender 220-640mm!!! It costs a lot to get 600mm on FF. Current EFS lenses are as good as the FF counterparts 17-55mm and 10-22mm were 2 of my favs. In fact the 10-22mm is hard to replace 17-40 is soft and the 16-35 not much different. The main thing I miss on FF is the reach of my 70-200mm it's doesn't feel as useful.
I loved my 40Ds and I kept one as a back up. Reason I sold the 7D was because I felt it wasn't a big enough upgrade, the difference IQ wise wasnt the leap I needed for my work. The two side by side IQ wise there wasn't a great difference apart from you could shoot to 12800 it's not great but you can if you needed to, but high ISO was much better than my 40D and gave way to a lot of opportunity I couldn't before. But at 100-400 it seemed noisier than it should, but the image output was twice the size so you can print big!
I found 3200 ok 6400 useable after a lot of PP, which meant more time at the computer.
But the camera a huge huge leap, screen res, features, AF incredible! In fact I have no idea how I used 9 points for so long. It's the king of APC and I would have kept it but wanted to put the money back into the 5D and I kept a 40D a very good backup and didn't need 3 cameras.
Heres a couple with the 7D, 1000 is pretty clean. I dont have any high ISO 7D stuff at hand.
IMG_8341 by
TomScottPhoto, on Flickr
IMG_8284 by
TomScottPhoto, on Flickr
Porsche Museum, Porscheplatz, Stuttgart by
TomScottPhoto, on Flickr
With the 5D I find 6400 good, 12800 useable after a lot of PP. So it does give you leeway and you get shots you wouldn't believe. 6400 I find about the same as 1600 on the 7D.
I made this as a test at 12800 at 2.8 with the 100mm L macro, they are very clean
BMW F30 3 Series interior by
TomScottPhoto, on Flickr
BMW F30 3 Series interior, light console by
TomScottPhoto, on Flickr
BMW F30 3 Series interior, centre console climate control by
TomScottPhoto, on Flickr
You can make incredible shots with any DSLR if you know how to get the best out of it. My fav shot taken with my 40D 17-55mm and a rig.
BMW Z4M Coupe by
TomScottPhoto, on Flickr
What I would do is learn more about PP and shoot RAW exclusively and wait for the new announcements, if they arent to your liking a 650D, 60D or 7D is where I would be aiming. Awesome cameras, yes older but you should be able to get them for a good price. The 7D will solve your AF problems and the 650D has all cross type. But all can be used at 6400 ISO with good PP.
Hope this helps.