GSM modems only handle a throughput of around eight messages per minute, so sending that number is going to take a very long time. Anything beyond the 100's and I'd certainly recommend using an online aggregator where possible (of course, this requires an Internet connection). Clickatell and IntelliSMS are both supported by FrontlineSMS (IntelliSMS is a little easier to set up). I don't think you're going to need to do anything with local providers either way, and even if you did it would likely be a long, painful and potentially expensive process.
if your only option was a GSM modem, then you can attach multiple devices to the computer, so in effect adding a second modem would double the sending speed, and so on. Three modems *should* give you up to 24 messages sent per minute, for example, but you would also need multiple SIM cards and you would then be sending from three different numbers.
I hope that helps.