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linux as suggested by aqua is spot on... if you require multiple ip changes for hunting / queries etc can also run multiple net connections... cheapest and quickest hardwall you can have! nothing compares! you can even run a mod on any old pc from 286 on, only requirement is extra simms or dimms to keep up with the net connection you may run
if you really want to spend any money on hardware (still prone to ddos attacks) try anything cisco... they were and still are the masters of security
We like Netscreens but the market has moved on since. You may be able to find something now with ASIC firewall like the Netscreen but also including features from other standalone devices such as Intrusion Prevention (sourcefire) and possibly QoS (packeteer) and deep packet inspection (toplayer).