The big differences are:
- what formats the player can play. Only the iPod will play music from the iTunes Music Store -- which does about 80% of the industry's business in legal music downloads. In addition, the iPod will play MP3s. Many iPods also can play video in specific formats (including TV show episodes, music videos, etc.).
- what accessories are available: cases, add-ons like FM transmitters and cables for car use, clock radios which can play music from an iPod, etc. The iPod is by far the most popular MP3 player out there, so there is a much larger accessory market. I dare say there's next to nothing out there for the RCA now and there will be absolutely nothing in a couple of years when that particular model is out of production.

MP3s can come from numerous sources. Millions of people download MP3s illegally. Many download from digital-music services like Rhapsody (but be careful: on many sites like that you only subscribe to the music and never own it -- quit paying for the subscription or watch the company pull the plug on it and your music library goes *poof*). Some Web sites (like NPR's and those of popular bands) offer MP3 files you can download and listen to. You also can play your own CDs/cassettes/LPs into your computer and send them to your MP3 player.

There are some other nice MP3 players out there. Creative's Zen is fairly nice. But many big companies, like Sony, Microsoft, and Wal-Mart, have tried to kill the iPod and have failed utterly. You may save some money by buying something other than an iPod, but you're gambling on the player being fixable when the battery dies or needs service. As far as I know, there are no other MP3 players which can play video. Frankly, considering the capabilities of most iPods relative to the competition, they are fairly priced and they work very well, with Apple's usual attention to how you interact with the device.

I don't see much point in buying anything else. Something else will be cheaper, but you'll be reminded of that in a dozen ways. Besides, iPods have that essential element of "cool" that RCA and other brands just don't have. There's someone else posting in this thread who bought Dell's player (another company which could not compete with Apple) last year and ended up buying an iPod this year anyway.

Good luck!