IDPA was formed in response to the heavy "equipment" focus of IPSC.
Both are competitions where time is measured to complete a course of fire, and time penalties are added for improper techinque, non-observance of rules, or bad hits.
IDPA restricts the type of modifications allowed with guns, and generally requires a "street-ready" gun. IPSC has new divisions that are very similar to IDPA, but others where there's a heavy emphasis on speed, rapid physical movement, large calibers, and high capacity magazines. Run and Gun...
But believe me: you don't win because of your equipment. You win because you are fast, accurate, willing to practice to excess (thousands of rounds a month), and highly focused. All the other folks shooting against you have comparable equipment. The standards are just different in IPSC.
I don't know anything about the new NRA type of competition, but suggest you go to the following websites for more information about IDPA or IPSC:
www.idpa.com www.uspsa.com