Minimum Ram for WPMU+ECommerce

Dear Sir,

I have windows 2012 VPS with IIS and MSSQL installed. It working good and ram usage is getting fit (1 GB remain avg.)

1) How much ram do I need more to install WP Multisite (3 sites) with MARKETPRESS ECOMMERCE plugin for all 3 site? (I have to install MySql too, right? )

2) Do you recommend MySql or MariaDB?