It really depends on how busy they are, part of which you can get an idea of based on what market you are targeting. Do you have a specific market niche you are wanting to target, if so how much dynamic content is on those sites and how busy are they?
John W, CISSP, C|EH
MS Information Security and Assurance ITEagleEye.com - Server Administration and Security Yawig.com - Managed VPS and Dedicated Servers with VIP Service
As a few people said before me, it really comes down to the resource consumption of the sites. Depending on your bugdet and expertise, I'd go either with an unmanaged 2 GB VPS(it's quite cheap and you have plenty of resources in case something goes wrong) or a reseller account.
cPanel HelpNet - cPanel, Linux, Virtualization tutorials, guides and more
If you already know the needs of those 50 clients, then exceptional could be only what fits their needs. There is no exact number of ram. You don't want to pay extra money for ram that nobody is going to use and you don't also want to start with complaints. My advice is to start by providing a hand by hand service to your clients and don't accept all of them at once. I guess you'll have only one box running at the beginning so you'll not be able to immediately move a really busy site that will have a negative impact on all the other accounts. Best thing to do is to get those 50 clients from your local market and build your business. When you'll be able to get more machines you could be open to international orders. Already said here...make sure your machine can get more power by upgrading and ask about times and availability of the upgrades. Running a single box...any change could mean downtime for all your clients...even an upgrade.
Another approach could be to get two ( or more ) "strong" vps for the money of one dedicated. So if something goes wrong with one, not all your clients will feel it! ALso this setup provides flexibility and gives you the advantage to make strategic moves so you keep everything balanced.