Currently I have three sites all running on a Access database and ASP. Now I'm planning on developing a new site which will have a high traffic rate and a database that probably will grow big.

Due to the restrictions of an Access database, the best alternative is obviously MySQL, also because it's free. Second I have to make a choice for a programming language. PHP is probably the most common language to use with a MySQL database, but the problem is I'm only familiar with ASP. Now my question is if it would be wise to learn PHP, or should I stick to ASP, or maybe even upgrade to ASP.net? I know PHP causes less strain on the server etc, but is the advantage that great that it would be worth it to learn a whole new scripting language?

Thanks everyone for their advice!