It is probably going to be best to purchase an additional VPS in order to make sure that you give all your customers the best and most resources possible. If you want, you can upgrade the RAM to start with, and see if that is sufficient to get you the speed and reliability that you need.
If you upgrade the 200 to 512 MB RAM, you would need to upgrade next to a 300 level plan in order to get more additional RAM. You are only allowed to add a maximum of double the base RAM included on any plan.
We don't keep to a strict number of VPSes on an individual machine, we rather keep each server's load and resources optimized based on the usage that the server actually is getting. Typically, there will be somewhere around 50 VPS accounts on a server, though.