By the sounds of it 500G is a monthly traffic limit, which isn't a lot, so I guess depends on the package that you got.
With regards to transfer speed, it could be many things limiting the speed to the server from your location. It could be server speed limit (US side), check with the hosting company, or if they don't specify it, try downloading something directly to your server from a location in US, for example use wget to get a CD image from CentOS US mirror.
It can also be your provider limiting your speed. Is it slow only for uploads, or both? If uploads are slow, but the downloads are fast it might indicate your ISP limits (they usually cap uploads to be x10 less than downloads)
And there are always intermediate links between your ISP and the final destination that could have limits configured.
As to the speed increase, I don't think there's much you can do about it. If it's your ISP, then see if you can get an upgrade to a better package.