A lot of VPS providers probably won't go up to providing 2GB guaranteed RAM, I don't think. DA shouldn't be a problem, it is fairly cheap to license with a VPS or Dedicated from your provider. I think your best off with a dedicated to be honest, up the budget to $150 and you may be able to get something.
Staminus will do an Intel Core 2 Duo 1.86 with 160GB hard drive and 1GB ram for $149 / Month. That includes 2000GB of bandwidth. For another $20 on top ($169 / Month) you can up that to 2GB RAM and have a nice system.
It may be worth it spending a little bit more to get a quality dedicated server, for example at Staminus - they've always been great for me and they have GREAT support! Also remember it's probably best to purchase something you can expand into, not just on your border line of resources
Are you using vBulletin?
You have 100+ users online in 15 minutes avg or 1 minute avg (cookie limit)?
Can you check how much queries per second is the database receiving now?
If you aren't serving files do you really need 1TB of bandwidth?