I used to have a vps with servint it had 256mb I had 2 forums 1 with an average of 50 users on at any one time, I also had 1 ecommerce site running cs-cart and about another 20 other static sites, I never had a problem with it, you just need to spend some time reading up about optimising it, I'm still with
www.servint.net only I have a deuce now because I host a lot more sites, but the backup and tech support you get from servint is second to none, give them a go you won't be sorry.