EDIT: I Bought the camera! 
Hi everybody!
I've been a frequent reader at this forum for a while now, but I never posted myself.
First out, I'm an amateur so I don't make any money off of my hobby.
I bought my first DSLR back in November 2006, a Canon 400D, and it's been with my up until this day (50k+ actuations).
I initially had the kit lens which I was pleased with up until it broke during a trip to Cambodia.
I later bought a replacement lens in South Korea, without really doing any research (I hadn't found this site at the time).
I ended up getting the Sigma 24-60 F2.8 DG EX which really opened up my eyes to photography in general and shallow DOF and low-ligh photography in particular. I got so hooked that I bought the Sigma 50mm F1.4 DG EX half a year later.
Now I feel that the body really is limiting me, since I LOVE shallow DOF and low-light photography (often combined). E.g. in-door portraits, pictures of food in candle-lit conditions etc.
Also, FF would be really awesome in order to get "more" out of my lenses in terms of FOV. Since the kit lens i had was 17-55mm it would be the equivalent if 17*1,6=27mm on a FF. My Sigma is 24mm, and on FF I would therefore achieve wider FOV than I ever had on the 400D before

I initially planned on getting the 600D, and then the 60D but I have realized that those will not really benefit me the way I want, since shallow DOF and low-light are my priorities. I never use my flash anymore. Sure, they have way better NR etc, but when doing an updrade after 5+ years, I want to take the leap.
So, the important questions (now that you've got my background):
How big of a revolution will this upgrade be, in terms of IQ, AF and well... pretty much every aspect?
I mean, it's a lot of money for me (I'm going to buy a used)
Currently anything above ISO 400 on my 400D pretty much ruins the picture, but I "have" to use 1600 in order to even be able to capture pictures in low-light, hence the upgrade. ISO 3200 looks great on the 5D MK II, so I really want that

This became a very long post, but I would really want to hear some thoughts... I'm thinking of upgrading within 1-2 months. Has anyone done the same journey? Will the lenses be a good combination with the 5DMKII?
My two current lenses are supposed to work with FF, so I will keep them

Btw, i could just get the 5D MK I and probably get way better IQ very cheaply, but I would like to experiment with video as well

Johan