I came from an XTi. For all of the creature comforts (AF, menus, features) it was pretty much a lateral move. For you going from one Rebel newer than my old one the only feature you'd be missing out on going to the older camera is the additional integration that the EX II speedlights bring in, but the menus are older looking.
I find the 5D to be on par with my Rebel as far as AF goes. The adverse lighting conditions that affect the Rebel more or less are the same conditions that affect the 5D. The 5D, like all other non-Rebel bodies brings in the ability to swap out the focus screen. I exclusively use the Ee-S precision focusing screen. That plus MTF lenses and the back button feature and all my focusing needs are met. The bigger viewfinder is a help too. If you've survived the 450D's autofocus then you should be just fine on the 5D. Moving subjects probably track a little better but it's not a huge difference. Wether that fits your definition of acceptable is entirely your determination.
ISO performance is about one stop (maybe 1 and 1/3) better than my Rebel. That is to say, if 800 is acceptable on my Rebel then 1600 will be just fine on the 5D. I'll actually use 3200 on the 5D, but for the most part won't touch 1600 on the Rebel unless I'm desperate.
My lenses are: 50/1.4, 85/1.8, 28-135, 40/2.8 The 50 was my go-to lens on the Rebel. I've been spending a lot more time with the 85 whenever I'm shooting with strobes ever since moving to FF. The 40 was purchased as mostly a toy for when I want things the be as compact as possible, but it is amazing when asked. The 50/1.4 is pure magic, pure absolute glowy magic on a 5D.