What kind of environment is your forum currently hosted on? Based on the usage there, we should be able to give you some more detailed advice.
50 concurrent users really shouldn't be a problem for a small to medium sized VPS. I'd say 384mb ram is probably sufficient - you could always upgrade later if you need more.
It also really depends on what kind of scripts you use, how well they're coded, and how well you configure your VPS. The configuration of your webserver and mysql (optimization) can also make a big difference. If you don't know how to do that yourself, then it's probably a good idea to get a managed VPS.
I use knownhost vps for my simple machines forum with daily peaks at 85-105, Have 384mb guaranteed but at this peak time it uses 400+mb of ram so im eating a bit of burst ram.
I use plesk 8.1, It is fast always and the peak times are just for about 1-2 hours and not everyday, the normal at any time is 50-60 users and it sits down on 300mbRAM. The forum is pretty active so it also uses a lot of mailing to the users with some threads subscriptions.
I also installed eaccelerator on the server and it helped a bit. So i agree 384mbRAM as Apoc told you.