You don't have to worry that much about your server power, i've got a P3 600mhz with 128 MB Ram, works as great as my other P4 2,4 with 768 MB Ram.
I don't think the difference will be huge, i haven't tried c++ on my systems, but you could use php & mysql. It's powefull, fast and stable. Like i said, i haven't tried c++ myself so i recommend it to try it yourself