OK, we've had this thread time and time again (usually from the same couple of people)...let's say this straight off...there are those "for" gear and those "against" gear.
I'm "for" whatever gets me the best results for the least money. Of my current gear, only my 30D is owned from new.
Now that we've got that out of the way...there are a number of occasions when you will need the large aperture fast glass and high ISO capable bodies. Sport is one of those times (and thank goodness - you mentioned it already!).
I was quite happy using my 350D and a 100-300 f/4 lens...I got shots published and they were good. Then, I started doing floodlit games, the ISO1600 combined with the f/4 aperture started holding me back. I got a 30D which went to 3200 (but was very noisy - however it was there if I needed it). Still wasn't entirely happy - I still wasnt able to freeze the action at f/4, ISO3200 and shutter of 1/250 (let's just say - Rodney Parade in Newport is a dungeon). So I saved, and bought a 300L f/2.8. Cracking lens, and it was great up until the 30D stopped working on ISO3200...then we were back to the same problem. ISO1600 and f/2.8 wasnt enough. I bought a battered 1DmkII which meant I could actually do some work. I then went on to get the 1.4x converter, which allowed me more focal length and reach with the 1.3 crop on the mkII. Then it became a problem as we were back to f/4. I wasnt happy with the background seperation for daylight games and the ISO performance for night games. So, eventually, I found an extra £1000, sold the 300L and bought a 400L f/2.8 for a decent sum of cash.
So, yeah, I spent a lot of cash on gear, BUT...when you're working in competition with other people and they have clean shots at f/2.8 with a lower ISO (and therefore less noise)...they will get the usage, and I wouldn't. Unfortunate, as sometimes my shots were better...but technically inferior.
Carp situation, but if you want the sales then you have to upgrade. If you're doing this for yourself, as a hobby - then by all means stay with the 350D and 100-300 f/4...it'll get you great shots and costs less than the mkIII body alone cost.