Something like your "RAM Disk Mode" may be useful in preventing malware from persisting on the machine long term, but will do little to protect you from today's malware from stealing information from you. Some malware will infect your browser through exploits, and harvest information you enter into other websites (such as capturing the username/password you used to log in here - or your bank). You would be amazed at the number of people (perhaps yourself included!) who will use the same username and password on an online forum that they use for their paypal, ebay, or bank account.
Most of the bad guys today are after credentials or financial information they can use for fraud, and they do this through software that seeks out and steals that data.
Other malware will infect you and look through that flash drive for any interesting information to send off to its masters (say a quicken data file, or a tax return, or an order receipt with a credit card number). It might not persist, but most damage is done within minutes of the infection.
Ultimately, you need to figure out the value you place on your information you may access with this computer, and the value you place on your time in cleaning up an infection or cleaning up a fraud attack. A paid security package will be your best insurance here, but there are some free versions that may meet your cost/benefit trade off. av-comparatives.org is a great site for independent assessments of security software, look for "real world" or "dynamic" tests which reflect the full protection of your system, not "retrospective" or "static" tests which are more geared towards brute force scanning.