How do I detect whether I have iPhone 2G,3G,3GS

Whether you are on an iPhone or a iPod Touch:

UIDevice *device = [UIDevice currentDevice];
NSString *systemName = [device systemName];

To detect the version of the OS:

UIDevice *device = [UIDevice currentDevice];
NSString *systemVersion = [device systemVersion];

To detect a specific model, you would need to test for some capability that only that model has, so to detect an iPhone 3GS, check for a video capability on the camera:

#define SOURCETYPE UIImagePickerControllerSourceTypeCamera

// does the device have a camera?
if ([UIImagePickerController isSourceTypeAvailable:SOURCETYPE]) {
  // if so, does that camera support video?
  NSArray *mediaTypes = [UIImagePickerController availableMediaTypesForSourceType:SOURCETYPE];
  bool isA3GS = [mediaTypes containsObject:kUTTypeMovie];
}

Here is a class written by Erica Sadun that provides extensive capabilities for this:

http://github.com/erica/uidevice-extension/blob/master/UIDevice-Hardware.m

Check out the rest of the repo - there are a few more classes that would prove to be really useful for fine-grained device querying.