Not sure why the above two posters are trying to add here. Your question was whether the Cisco 5505 helps against DDoS attacks and technically the answer is no.
It can help filter depending on the rules you add and decent for DoS attacks but it's merely a firewall. The common misconception about DDoS protection is that it's a firewall. Yes, you do blocking, but it is normally in the form of dynamic blocking based on granular packet or deep packet inspection. Products built for DDoS protection are usually geared at that function / IDS functions entirely. It requires a combination of packet inspection (anomaly detection), known attack methods, and rate limiting.
You won't find a cheap (<$2-3k) *purchased* solution for real DDoS protection. You can find things that will assist with protection but not provide protection that would hold up against a large attack.
If you want *REAL* protection then you'd need a custom solution or a high-end commercial product such as:
Intruguard / ArborFlow Product lines
Mid to High range protection would be something like Cisco Guard/Netscreen/TopLayer appliances.