@Wendo:
I may be missing something here, but why do you think you need 2.5Ghz + to run a 7Mb DSL connections?
I can max a 15Mb cable connection with a 500Mhz Pentium 1
If you have more than one internal subnet and have them segregated using different interfaces in pfSense then traffic between the subnets (and hence the interfaces) is filtered just as it would be between WAN and LAN. This if both 'LAN' interfaces are gigabit you will need to filter at gigabit speeds.
@sully: If you can't have segregated internal networks, because of your existing wiring, then you don't need fast box. Even so I can get throughput of around 500Mbps from my Pentium-M 1.5GHz box. It would be faster if it had nice Intel NICs.
If you think about it carefully you can probably come up with some compromise solution. E.g. put your teamspeak boxes on a separate interface close to your pfSense box. Leave your NAS in the same subnet as your clients so that traffic is not filtered.
The concept of red, green and orange interfaces doesn't really hold true for pfSense since, especially in 2.0, all interfaces are treated equally. The differences between them are simply down to what rules you have applied. E.g. you could have five internal interfaces and they would all be 'green'.
If you are going to use pfSense as a web proxy it would probably actually be easier to use it as a router as well. ;)
Steve