I would go for sql server 2005/8 in a .net enviroment.

I have never had any speed problems. Also if you are going to use asp.net membership stuff it makes sense to go with it. There is a mySQL membership provider but its not great from my playing.

Also the cost is not going to be that much because you can do an aweful lot of processing with one MS database server.

I am sure with some googling you can find some bench marking of all major dbs.

I use discountasp.net for my .net apps as a starting point.

One last thing, finding someone to work on your MS sql database might be easier.