Pure DSLR IQ Image Quality
I do not own any of those 3 cameras. I have a E-520 and an E-p1 ( same sensor as the E-30 & E-620 )
Those 3 cameras are different beasts as you might've figured. For pure IQ I think the E-30 & E-620 are widely thought to be the same so pick the one that suits you best ( compact (620) vs better viewfinder/slightly better autofocus and features ( e-30 ) )
The E-450 was an interim camera that was an upgrade on the E-420 with the same sensor as the 420 but slightly improved imaging processor than the 420 - but I have not seen many examples from this camera but is again thought to be not a match for the 620 at higher sensitivities & for DR ....
One IQ advantage the 450 could have - could come from the same factor where my 520 wins over the 620 which is that it has ISO 100 (whilst 620's base is ISO 200 ) and is super clean whilst the improved DR in 620 is thought to be a bit of tone-curve magic that renders the shadows with more noise .... and I think this is true as I can see evidence of this in my E-P1 which is hands down better than the 520 in most places except base ISO - the 520 on a 'good light day' can do a better job with the shadows.
hope this helps