Back to the OP.
If you like shooting sports and don't have a lot of money to spend on long glass then a crop camera is what you are going to want. If you're after Canon then the 7D fits the bill perfectly. It's a very versatile camera which will perform all the tasks you require of it.
If you want to go full frame then you may need to be using a TC with lenses (which means buying a fast long lens to start off with) or paying the money for longer glass. It may be though that a 100-400 or 150-500 type lens might give you the reach you require? If so they're not too expensive.
The 5D2 does have better IQ and ISO performance than the 7D, it's AF is not as good though. The outer points are not cross type and in low light struggle to lock on. You can mitigate this slightly by always aiming for high contrast areas in normal use but in low light it'll be hit and miss. The IQ is very good and 21MP gives you a lot of pixels to crop as well if needed.
Then there's the D700, I've been looking at this myself of late as the AF on this camera is very good. I don't know enough about the Nikon glass at this stage to make the change myself though. The 7D and 5D2 both do video as well if you are interested in that, the D700 doesn't. It also does have a lower MP count but that really isn't an issue unless you want to print really big!
As you're into sports the 1D3 should be mentioned. Cost wise it's around £1250
second hand. The AF is very good, it's a 1.3x crop and has good IQ. It's only 10MP but it's a very good 10MP. Personally I rate the IQ slightly better than the 7D.
You could also look at the 1Ds2 but this is not a camera up to todays standards of ISO and user interface. It's a heavy old lump with a kooky 2 button interface. It does however have great IQ and AF and I love using it. With LR3 it's useable up to ISO3200 as well.
You should also bear in mind that a lot of the comparisons with IQ come down to actually quite small differences that in the main only 100% peepers will see. For web sized (1024x) and printing up to A3 size you'll most likely not notice a great deal of difference. The shallower depth of field from a FF is nice and it does let you throw the background out of focus just that little bit more than a crop, and I have got shots with the 1Ds2 that I haven't with my 7D no matter how hard I try to replicate them.
but it's pretty expensive too.
