Hi Di, welcome to the Cameralabs forums!
I notice you've not mentioned the Nikon D80 in there? If you're concerned by the performance of the Canon and Sony DSLRs and want a step-up from them, then the D80 is the way to go. The D40 is also pretty neat if you want an entry-level DSLR. Our review is here:
If you want the widest selection of lenses, you should be considering a Canon or Nikon body.
But if you stick with the A100, the twin lens option you mention with the 75-300mm is sensible. There's a review of the 75-300mm here:
If you're after a step-up from the Sony kit lens in quality though, it could still be worth waiting for the Carl Zeiss 16-80mm, although I've still not seen one...
If you can wait to see if the results are god, I'd also seriously consider the Olympus E-510 - see:
http://www.cameralabs.com/analysis/Olym ... ympusE510/
Oh dear, I appeared to have given you even more options to consider!
Have you been into a store and picked any of them up yet? This can decide it for you straightaway.