Not sure why you'd be worried about getting DDOS'd, unless your sitting on a routable class-B or larger it's very unlikely unless you are messing with the wrong people.
As far as hardware unless your doing Snort or Squid you really don't need that powerful of a box. If excluding those to options I've held up 100MB business links running pfSense on boxes you would be throwing away these days. cough cough P3 933mhz + 512MB ram, granted more modern releases of FreeBSD are slightly more resource intensive, I've had no problems running it currently with 1vCPU and 768MB ram under VMware….Yes a VM in production and it works just fine.
CPU:
If your really worried I'd probably go with an i3-3220 which is likely overkill (G2020 should be good enough, really looking at the 55w TDP)...if your doing Snort at line speed it really depends on your WAN link. On gig+ links with 2000+ clients banging away at it your looking at westmere xeons unless you want to do some port-mirroring.
Ram:
4GB would be plenty for most things and cover you down the road. If you are planning on Squid then 8GB, but make sure the motherboard can take 16GB down the road in case load increases.
NIC:
As tirsojrp said, pickup a used dual port Intel Pro1000/PT PCI-E adapter off Ebay....should be $30 or less shipped. A lot cheaper than new, and a lot higher quality.
Storage:
CF works but I would go with a regular USB thumb drive, USB2 drives seem to boot quicker.
PSU:
Always get a high quality PSU, being cheap can cause all sorts of issues from higher failure rates to odd voltage outputs and fluctuations.
Worth mentioning as no one else has asked. "Huge LAN parties...will go on the fritz", what kind of switching is he running? I wouldn't recommend running 100 seat lan parties off netgear switches, no offense they have their place but when you care about latency and have a large network...I would go with something of a bit higher grade.