what's an xsi in real numbers? 450D?
ok, it's an older body, and *only* 12 MP. But for what it does, it's great.
you've already got the 'good' kit lens in that it's the IS version.
the 70-300nonL is good for its price (but i personally hated it at 300mm and got the L).
so going from 18-55 to 24-70 you will:
a) future-proof yourself for fullframe.
b) get a big difference in IQ.
c) gain 750g to your kit.
d) lose IS, lose 18-24mm (which will hurt on a crop-body)
e) gain a few stops of light, gain 55-70mm range.
going from 70-300 to 70-200 2.8 i:
a) lose the 200-300mm range.
b) maybe you've got a good copy, maybe i had a bad copy, but i wouldn't miss the 200-300 range because it was too soft to use.
c) if the deal comes with the extender, then you gain the 200-400 f5.6 range instead.
d) go a bit up in IQ in the 70-200 range, and gain 1-2 stops.
e) gain 680g in weight
f) Lose IS? you didn't mention if it was the IS or non-IS version.
going from 450D to 550/600/60/7D, depending on model:
a) definitely gain 50% more MP
b) maybe better AF system
c) maybe bendy screen
d) maybe more FPS
e) definitely video
So, if you upgrade the wide lens, you still have to swap to the 18-55 for the widest shots, or get a 10-22 and stick with APS-C. If you can put up with swapping lenses for a while (up to a year or two?) then upgrade to 5D3, go that.
If you're always going to be 7D or less, ie APS-C, then you'll definitely want to pair a 24-70 with a 10-22 (or equivalent 3rd-party). Or consider another option, 17-55 2.8 or 15-85 (choose between these on zoom range vs low-light only, hardly any IQ diff).
If you upgrade the zoom lens, and go the 70-200 + extender, take it out in the field if you can first (or rent one?), see how many shots you miss because you were too busy putting the extender on to get that extra reach. if it's the non-IS, see how much that affects your shooting style. see how much better the photos are wide-open at f/2.8. Also consider the 100-400 as AJ said, or a bit cheaper now is the 70-300L, better IQ than a 70-200+tc (how much better depends which flavour 70-200), no changing TCs, if you can put up with the f/5.6 for extra reach or convenience.
if you go the body option, one thing to think about:
the xxxD models are upgraded every year on the dot. they'll take the latest sensor whatever it is. if the 7D2 comes out soon, and at least 6 months before the 650D, then the 650D will get that sensor.
(it might not, but it has happened every time until now. even the 400d got the sensor *before* the 40d, the 50d sensor went to the 500d 6 months later, and the 7d sensor went to 550d 6 months later).
you can wait on a body, but every time there'll be something new or cheaper just around the corner to wait for. bodies degrade in price a bit more than good glass does too. if you're buying a used 70-200 2.8 nonIS, then factor in that if you ever need to sell it, and treat it well, you can probably get back almost what you paid for it. not so with a body.
hmmm, i came here thinking that the obvious answer was to go the 24-70, but i'm not so sure now.
i think the biggest question you need to answer first, is are you going to be definitely going FF at some point. the 5D2 will always be a good camera, regardless of if/when the 5D3 comes out. my bet is the 5D3 will be 50% more than 5D2's price now on release, a used 5D2 will hold its value for a while until 5D3 stocks are ubiquitous, i might even consider one at that point. upgrading the wide makes no sense if you're staying with APS-C, or get an APS-C wide (17-55, 15-85). So my money's actually on going the long-upgrade (also consider the 100-400 or the 70-300L), then if you ever go FF you'll have to get a wide-angle FF lens at that point too...
and i didn't give any good arguement as to why not to upgrade the body. my feeling is that the new models are coming out *soon*. but 12 MP is more than enough unless you're doing enlargements, or plan to crop out smaller bits when your lens can't reach far enough (i do sometimes). I'd stick with 450 until 7d2 or 5d3 some out, then maybe even a 2nd-hand 7d or 5d2 will be very attractive once the price has settled, a newer model doesn't make the old model any worse than it is now...