1) Out of all my gear, I "regret" very little.
If anything, it's minor stuff. Like I kind of regret paying something like $65 or $70 for a 512MB CF card out of a CompUSA last summer (when online places like B&H had the high-end 1GB cards for $90) but I needed it at the time.
I sorta regret buying the Lowepro backpack bag- not that it was a bad bag, I loved it, it's just that after almost a year, I realized I'd never once actually used it as a backpack. I sold on eBay for about a $20 loss and bought a Tamrac Pro 8, which has worked out much better.
After buying the 28-300IS, I sort of regretted having spent $125 on a used 75-300 f4-5.6 (they're $150 new) but I'd bought it and used it for several months before I could afford the big L, so it proved useful. And now with the 1D, I have two seperate bags; one with the "high end" stuff in it, and a much smaller, lighter one with the XT and the inexpensive stuff. I can grab the little bag if I think I'll want to take snapshots and informal stuff (family gatherings and such) or I can grab the big bag if I plan on doing some "real work". 
Other than that, my collection isn't all that big. I haven't bought or sold much that I didn't research, plan and ask about, considering what it took to save up for most of it.
As for #2, the XT, hands down.
Yeah, I bought it back when they were pretty new and no rebates (just under $1K plus accessories) so I pretty much paid "full price", but it was worth every penny.
It's never given me a single problem over the course of well over 25,000 photos, it's given me great images, and it was light and small enough to carry everywhere. It helped me relearn my nearly-forgotten high school photography class education, long atrophied by nearly half a decade of point-and-shoot digitals, and got me interested in photography again.
I still use it for snapshots rather than dragging the 1D out, since it's smaller and lighter, and at the moment, the 18-55 is more useful than the 50mm 1.4 as a walkabout lens. (Though I plan to get the 24-70 for the 1D, which, interestingly enough, is almost exactly identical in range on a 1.3 crop body as the 18-55 is on the 1.6 crop.)
On the low end of the "best bang of the buck" scale, I bought a $19.95 four-section no-name (Chinese import) monopod from the "photography" section of a local big-box store. (You know, where they have a bunch of point-and-shoots, camera bags the size of a kid's lunchbox, and memory cards labelled as the number of photos they hold rather than a "-byte" capacity.)
Surprisingly enough it's sturdy, the locks work extremely well, it feels solid in use, and it's more than tall enough for me. It can and has easily held my 1D and 28-300 without any problems- and that's what, eight pounds or so?
Well worth twenty bucks. 
Doc.