i managed to find a report from Buckinghamshire CC http://www.buckscc.gov.uk/assets/conten ... survey.pdf
which actually compares measured pH and stated. The sparkling water is all around 5. About the same as coffee.
The reports re dental errosion are very confusing and some actually say that mineral water witha high mineral content can be good for dental enamal irrespective of carbonation.
Personally i just think that it is not natural to drink a lot of fizzy water so it can't be good for you

Sodium is allowed in tap/bottled water up to 150 mg/litre but I think it is usually much lower, around 10-15. So as Snoopy said, not a worry with a recommended 1600 mg/day.