Stressing out db connections

I have worked on many small sites and now time is calling me for a high-usage site. Right now, we have a PII 400 with 64 MB PC133 RAM on Redhat 6.2 which will be used to jump start the site. What I want to know is how much connections to the database can be handled at once on the server, or perhaps some tips with the hardware architecture or software architecture would be much grateful. The site is a forum based site with over 1 million members at the start and increasing rapidly. What I also want to know what DB would be more suitable for the job, Oracle or MySQL? I have both and have written much of the forum software in MySQL which I don't think will be a big tuffy to convert over to Oracle. I hear that Oracle takes a great amount of RAM though just for the installation. Yet, in the long run, will Oracle should be my choice to power multiple boards with multiple users connected to a database simultaneously? Thanks for any help or insights you can give to me.