RAM is a key resource to look for and decide upon in a VPS. With shared hosting, you only have disk space and data transfer to worry about, for the most part.
A shared hosting account is like toast in a box. The size of the box is how much RAM/CPU exists. Movement of toast within the box is resource usage by the toast. Your web site is the toast and the server is the box. Shake the box and all the toast moves around and bumps into each other, sometimes not fairing so well, but there is plenty of room to move around in the box.
A web site on a VPS is like toast in a smaller box within a box. The outer box has several smaller boxes put inside it and the toast is placed within the inside box. Now, the toast from each inner box no longer bounces against the toast in the other inner boxes, but it has less room to move around. If you don't want your toast to touch other toast or you have special toast needs, this is a good thing. Selecting RAM and CPU resources for a VPS is like selecting the size of the inner box. It all depends on the toast. Small cubes of toast will fit better than a loaf of french bread, and so on.
Burstability (RAM or CPU) is like having an expandable inner box that always returns to its original size when there isn't enough room in the outer box.
When it comes to RAM, more is generally better as long as it is properly managed. Purchase a VPS that has as much guaranteed RAM as you need and keep the burstability in reserve.
In your case, 20 to 25 forums (typical usages) will probably do okay on 256MB guaranteed RAM if you're not using CPanel or Plesk. It all depends on the toast (err forums, in this case), though. Are they heavy usage forums or lightly used? Heavier usage requires more RAM.
I consider a forum to be heavily used if it typically has 20 or more users browsing and posting within a 15 minute period (someone is bound to disagree with me on that).
You can also somewhat tell by the sizes of the databases for each forum. Under 50MB is usually light usage, but it somewhat depends on how long the forum has existed and what types of posts are typically done.
If you just don't know, ask your current hosting company to rate your account's usage activity as high, medium, or low and go with the next lower level. Most hosts are going to tell you that your sites are more active than they actually are because they have an interest in seeing you upgrade.