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.