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As you've already discovered, a 3925 is a little too small to handle a busy gig link. For just one gig link, the recent 4500-X would be better sized, but for two gig links, you'll probably want to consider the lower end of the ASR 1000 series rated at 5 Gbps or better.
BTW, regardless of the router, some DDoS attacks are not easily dealt with. I can easily send x amount of packets toward any IP. They can be filtered, but it doesn't stop the bandwidth consumption before the filter. I.e. you could protect the web server, if you can identify the traffic as invalid, but such an attack may still consume 800 Mbps of your Internet bandwidth.
