As stated earlier, the problem may not lie soley within your connection. It may be due to your low end system. Contrary to this, I know many people who can run MOH:AA on worse systems than yours with realitive ease. So, in essence, a good chunk of your problem is caused by your connection (or lack there of :P) Two sites were posted above which you should check out, and another great one that he neglected to mention is:
http://www.speedguide.net

This provides many tweaks to allow for a slighty faster connection. But, sadly, most of these tweaks that we speak of do not make a signifcant difference to your ping. (If any.)

As for the reason why your system is crashing every 15-20 minutes, I can only come up with only one possible hypothesis. Most servers run on a map rotation, and I know from personal experience, that even with my cable, sometimes when the server switches maps there is a long period in between. Probably caused by paket loss, or the like. And with your 56k, these gaps may be longer. Maybe you system didn't crash per-say, maybe you just have to wait it all.

That's the price you pay for 56k.