i have both types of mp3 players: HD and flash.
for bike rides, i just take the flash as it is lighter and smaller.
I have the iRiver u10 which is 1gb in size with a color screen and capable of photos/videos/FM radio/audio recorder... not that you'll need all of those things for exercising.
I think iRiver also has a 512kb version as well as the new 2gb version.

it can be had relatavely cheaply at eBay