Exactly what I'm using now, average around 5% CPU usage, 700MB/2GB RAM, running squid, squid filter, snort, and clam-av for about 16 devices. Awesome stuff, probably take a hundred devices to bring this to its knees
Many manufacturers allow overclocking on h97 motherboards now. It depends on the BIOS of the board thoughhttp://www.techpowerup.com/202196/asus-enables-overclocking-on-h97-h87-b85-and-h81-series-motherboards.html
I've had trouble running a 4850 and a 6870 together at the same time. Mainly because of a driver conflict because AMD stopped supporting the generations before 5000 series so the drivers aren't compatible with each other.
I would just picked up a cheap 5450/6450 or use the integrated graphics