20-25 people is not a big deal
IF everybody is not in their own personal dimension. That will be one of the biggest strains on your server if everybody has their own personal mystcraft age.
Somewhere in the range of 3-4 gigabytes of RAM
maybe 2 gb will be good for memory. You're going to want a fairly powerful cpu processor, over 3.0 GHz would be helpful. I would say a 1Gbps upload/download link would be what you would need for that many players with all the mods especially if you're going to have multiple worlds, since a lot of mods stuff the network full of events.
You're looking at a budget anywhere from $18-$40 a month, dependent upon who you go with and what you decide to buy.
The only thing that will come second to that is chunk generation. Vanilla minecraft by default, completely sucks in terms of properly utilizing hardware. With 20-25 people you should probably look at something like this
http://ci.md-5.net/job/MCPC-Plus/
That would be your minecraft_server.jar or whatever the ftb pack names its server jar files.
You may or may not need a dedicated server. I run a server with about half that amount of players and everything works fine no lag except for on chunk generation.