Sigma vs Tamron vs Minolta

I am about to get my first digital SLR and don't have much choice as I own
Minolta lenses with the odd Tokina and Sigma lens thrown in. I think the
lenses I have from 70mm upwards to 400mm should be fine for the 7D but have
to decide on a lower end zoom, say 18 to 70mm (round-a-bouts), as the CCD as
you all know is the APS size. But I can't seem to decide whether to just
go the basic Minolta lens 18-70mm which has been made for their digital SLRs
or to go one of the other two brands. There is one Sigma lens which seems
to fit my needs which is the 18-125mm zoom with an aspherical element, worth
about $AU369. Not sure about Tamron as they seem to not have a straight
forward zoom around this specification at a reasonable cost for my budget?
Trying to keep the total cost of SLR/one lens/512mb card under $2000, but
much prefer the 7D over the 5D for looks, feel and function. That's not
easy, but thanks to the $300 cash back Minolta offer currently, it'll help.
Does anyone have any experience with these lens brands? I have a cheap
Sigma 24mm fixed lens that I bought 10 years ago and it has nasty blurring
at the frame edges, so that experience isn't helping but sure technology has
moved on since. My 400mm f5.6 Tokina and 70-210mm Minolta are fabulous
lenses.

Anyway, thanks in advance,

Jasen