Depends entirely on what your 150 accounts are doing. If they're serving static sites, less than 1GB will probably do. If they're running PHP applications and actively making extensive use of CPU and RAM, you're going to want considerably more than that to maintain satisfactory performance. And if your customers demand excellent performance, even more.
If you were using shared reseller hosting before, bear in mind that those kind of machines often have dozens of GBs of RAM and loads of powerful CPU cores. You might find a VPS suddenly very restrictive if coming from a generous shared reseller provider.
In other words, expect your budget to go up considerably.
According to me 1 GB. However, there is no hard and fast rule for multiple accounts. As, "Ryan Williams" mentioned that it generally depends on application used by your clients. If they are using heavy scripts, plugins, tools then you will need more Ram to offer them satisfactory performance.
Personally, I think 1024MB is going to be alittle on the small side as you need room to move and you dont want to be bursting everytime abit of sudden traffic happens, I recommend not using anything below 2048MB (2GB) Ram.
According to my own testing a stock install of cPanel on CentOS will use this:
cPanel/WHM - 214.68 RAM USED - 2.88GB DISKSPACE USED
Note: These maybe be different for you!
Last edited by Server Management; 05-16-2011 at 07:53 AM.