The 500/4.5 is certainly one of my favourite lenses... although the new price isn't as attractive as in the last year or two of the non-DG version, which saw it at £1995 or even lower. 
I may be wrong, but if you put a DG teleconverter between a camera and a non DG lens, then you effectively get the (debatable) benefits of 'DG'. The thing to avoid is non-EX models of the 500/4.5 (and 800/5.6.)... EX makes a big difference and is what raised Sigma's profile in serious photography. 
  
There's a mint Nikon non DG for £1399 at Surreyphotography. 
  
Ultimately, a Nikon 400/2.8 (has to be AFS, the older ones were very slow focusing) would seem the best bet for action sports... but they are only a fraction smaller and lighter than my 600/4 and the 500/4.5 is tiny in comparison 
See photo below. 
  
btw: my 600/4 is currently on ebay if anyone fancies it.
		 
		
		
		
			
		
		
		
		
		
		
			
		
		
		
		
		
	
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