I currently shoot with a T3i
That's a good camera with a solid sensor. I was considering in when I was looking to upgrade from a Canon XS. Ultimately I went with a 60D for performance (not picture quality as they are about equal) and the feel and ergonmics... and the cross type AF points. If you don't need satellite AF points, I'd stick with the t3i. The t4i would be a marginal improvement and eventually you would question whether it was worth the upgrade.
50mm f/1.4 is a great lens for the money and performs as well as the L version (though people will debate that till they are blue in the face. Sell the 50mm f/1.8. As a backup? It is a fine lens at f/2.8 and higher, but it isn't worth the lack of money you are able to put into another lens.
18-55 kit lens is a fine lens, but when I had that paired with the 55-250 and the 50mm f/1.8, I rarely used it. There isn't much of a resale value for it, but I'd suggest wrapping it up in the original box and wait until you are ready to sell the t3i... then the lens will be in mint condition.
The 17-40 is a good lens, but not great. It doesn't really sharpen to the point to where it resembles an L series lens until around f/8. That's great if you are tripod mounting it, but if you are hand holding, it leaves quite a bit to be desired. Iso goes up and your shot gets grainy, or the shutter speeds are too long and you might be a touch of unwanted blur. It also costs around 600ish if you pay tax. I'd suggest taking that 600 and buy a 24-105 from bigvalue for around 750 when they have their ebay sale. Even if you are paying 800, that is a solid upgrade at a GREAT price. It's not a perfect lens (vignetting on full frame, not super sharp between 85-105, etc.), but it is a very good lens and it still compares very favorably for the money to a 24-70 (any version). The f/4 will be fine if you are shooting in sunlit areas, and while f/2.8 provides a prettier bokeh, it sounds like that might not be your main concern at this point for a kit lens. People claim the 24-105 is really sharp around f/4.5... and I won't argue with that.
I like the 55-250, and I think it is a great value lens, but I've been leaning towards not using longer zooms. Keep it for the zoo, but you'll notice that the 24-105 is sharper and gives a nicer image even if you have to crop into the image v. the 55-250. I'm not saying it is expendable, but you might have a hard time taking the 24-105 off in favor of the 55-250. Sell it for 150 and that combined with the 80 ish from the 50mm, and you have the money for the 24-105.
... like the 5d mark ii, it really makes me rethink. Do I need to take the plunge?
Go nuts, but your better off investing in better lenses before you invest in a really nice body. You would basically have a really nice engine with a mediocre transmission. Both have to work together in order to get real performance. Having said that, the 50mm f/1.4 would be a good pair with the mkii. I like primes, but I'm not sure I want to go back to mainly using the 50 prime again as my main lens. Shoot the 50mm at f/2.8 to f/4 and it is one of the sharpest lenses you can get... even sharper than the f/1.2. None of your other lenses would work with the mkii save for the 50mm f/1.8, but again... I'm not really sure why you have 2.
I wonder if the jump to a higher level cropped sensor is even really worth it. It can still take impressive quality images (in my opinion):
Don't get another cropped, until they substantially upgrade the sensor. This sensor has been basically used in the t2i, t3i, t4i, 60D, and 7D. It's crazy how old this sensor technology is in new tech.