I can vouch for the 7D as I own one myself and have a 50D as my backup. I had an XTi for quite some time and I shot at 800-1600 and never felt that the noise level was out of control. And once a little noise reduiction was applied in lightroom, the pics were good to go. Of course, it will not hold an candle to the newer bodies as far as high iso-low noise goes but IMO it was very acceptable. You have heard about the firmware upgrade to unlock higher ISO’s in the XTi right? The XTi and 40D share the same sensor and with the upgrade will have similar noise levels as a 40D.
Also, again off topic but using a 17-50 and a 50mm lens as you two lenses is kind of silly being that have have two lenses at the same focal length. Waste of the second body really. I do realize the prime can open up a lot more then the 2.8 of the tammy but realistically taking pics at a wedding at anything less then 2.8 is rare. I’d say have the tammy on one and a 50+ on the other (ie, 70-200, 55-250, 70-300, 85mm, etc). For wedding I use primarily my Canon 17-55 f/2.8 IS and my 70-200 2.8 IS and the whole thing could be done with just those two. I do occasionally throw on the 10-22 for some wide angle shots of the venue’s or maybe a group shot of the entire party.
I am NOT against upgrading bodies, as I moved my way up too. I just don’t like seeing people upgrade when it’s not completely necessary. As you know, lenses first! Lol. And in all honesty, I did a wedding on Monday and I don’t think I went over ISO 500 the entire day, until some special firework shots (which were at ISO 2000). Flash is key!
Side note. Will you sell body only? Lol, I may pick this up for a third body.