You need something with power in order to sustain dos-attacks. I'd recommend a Foundry Bigiron or Netiron. If you don't need ipv6 and don't want to carry more than 500 megabits a ironcore-B8GMR3 module with 256 MB will do (256mb can hold up to 400.000 routes, so it will take a while until you'll have to replace the module).
Otherwise a newer gen4 jetcore module with 512mb ram will do a pretty good job even under high load.
Gunther