To the TS, why do you feel its time for you to upgrade to FF? What is it that FF offers that your 400D cannot give you? Maybe if you can tell us that, it'll be easier for us to help you on your decision.
Buying a FF camera doesn't mean just paying for the camera and the kit lens. You might even need to buy a whole lot of new lenses as FF cameras expose the mediocracy of cheap lenses very easily and obviously. You will need to change your lenses to decent ones, or even L lenses to enable the FF camera to achieve its fullest potential by matching the equipment. Ultimately whether the photos are nice or not still depends on the photographer and his skill. Someone with a 2megapixel old PNS can have better photos than one with a FF camera if one lacks practice, and lacks the creativity. The camera plays a part in your photos, but a small one.
Also, most FF canon cameras produce files which are very huge and takes a powerful computer to process them efficiently. If you don't already possess a computer with sufficient processing power to process your photos, then you'll need to upgrade your computer as well, thus incurring extra costs. You will also need more/bigger and faster CF cards to store the huge files from your camera. Once again, extra costs. Maybe you'll also need to upgrade your harddisk to accomodate the ginormous files you're going to be collecting with your new FF camera. More extra costs.
Canon FF cameras do not come with a built in flash, and thus you will most probably have to purchase a flash to go with your camera as well, and this, is another additonal cost to your expenditure.
When you buy L lenses, especially f2.8 and above, you will definitely also want to pair it with a B+W filter to optimise the light going through the lens. And we all know here B+W filters are very good, but not exactly the cheapest things around. One more additional cost.
If you get the camera, and because you have already spent so much on your lenses and your camera bodies, you'd want to purchase a dry cabinet to store them, and a good camera bag to protect your equipment while transporting them. All these are again, additional costs on top of your camera kit assuming you don't already have these.
So as you can see, buying a FF camera is a huge capital outlay, and is more than just buying the camera kit alone. There're costs involved in maintaining, optimizing performance, post processing etc. And if you're taking photographs as a hobby, it is a HUGE investment with non-monetary returns unless you start taking jobs (and it'll take you a while to recover your costs).
So aside from costs, the most important thing again, is WHY do you feel its time for you to upgrade to FF? Do you really NEED to upgrade to FF? OR do you even need to upgrade for the moment? The 400D is not a bad camera in itself.
Just to share, I only upgraded my body after 3 years of shooting with my 350D, which has seen me through learning about DSLRs, to hobby shoots, to covering events, to covering paid event jobs, which eventually paid for my 5DMkII which i bought recently, for taking on more assignments. And of course, along the way, i collected a 17-40, a 24-70, a 50 1.4 and a 580EX, all paid for by taking on assignments with a 350D. Maybe you could spend the time learning/practicing/saving up/earning some money to finance your own equipment also. It gives you a sense of ownership and satisfaction when you finally exchange your own hardearned money for your new gear.
Just my 2 cents worth. Quite worth it for you, because the post is so long. Haha!
