If you allow people to enter cc information on your web pages directly, then you probably should have SSL to keep things secure.
However, you can use paypal or other credit card processors (some of which are specifically setup for donations) that don't require the information to be entered on your website, but rather on the processors site. This elimates the need and expense of a the SSL certificate.
See
http://www.clickandpledge.com/ and others. Paypal is nice since it doesn't have signup or monthly fees, and you can setup 'buy now' buttons to purchase memberships, or donate, etc. (Though I have found some that shy away from paypal)
-Tim