I just picked up a used 21.5" with 4MB Ram and still under warranty (so I can add Apple Care) for $750.
In theory, the extra ram and Apple Care should add $150-$200 to the value. Unfortunately, buyers often discount the value of add-ons. Your computer may not sell for more money, but it might sell quicker than a used base model.
As a starting point, you can check Mac2Sell, eBay (including completed sales), or Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc.
with the extra RAM and extended Apple Care, you can fetch 800-900 tops. Factors as cosmetic/functional issues, original packaging and accessories and general care of the machine also counts to go up or down with the price.
Also, if selling in an online site, factor in shipping costs/fees