Customize your feed excerpts by simply using the <!--more--> tag in posts.
If you don't meet the below requirements, I highly recommend you upgrade your WordPress install or move to a web host that supports a more recent version of PHP.
/wp-content/plugins/directory and unzip
Once the plugin is installed and activated, go to 'Reading', which is under 'Settings' in the WordPress admin menu. Make sure you are using the 'Full text' option for your WordPress feeds. Now, just use the <!--more--> tag in any post to create the cut-off point for the feed excerpt!
NOTE: To customize the link text, just add your text to the more tag, like this: <!--more My custom text...-->