I haven't used Interserver in some time; but, at
StableHost (affiliate link), my price is a little more per "slice", but each level is an additional CPU core. So for my $29.95/mo 4 GB/ram 4 core VPS at SH, Interserver's equivalent would only get you 2 cpu cores, but 5 gb ram (it's been a while since I made the comparison, it seems like they were more in line a while back).
As times go on we find ourselves needing more resources, on a VPS or a Cloud Host we can scale up relatively easily (note that cloud products have their own caveats), but for purchased dedicated machines people end up having to invest quite a bit more.
I'm happy to help, but unfortunately server configurations are really out of the scope of the support forums for a web script... I've seen similar sites fly with similar resources (~6gb ram, multi-core), the only thing I could suggest is that you monitor your mail logs on the server and see if anything is out of the ordinary.
UBB.threads is a web script that runs on your web server, it'll use whatever resources it needs to run on your server; a busy site with lots of content will use a significant amount of resources. If these resources aren't available one will generally need to upgrade their system resources and tune the server accordingly (such as configuring a new web server, like your host has done).
It seems that your host is trying to show you that the identified heavy system resource hog is the web server, which is running UBB.threads, and they're trying to identify ways to bring resource usage down (which I doubt they're going to be able to do, since it sounds like you need more resources to tame the web server usage).
