as for the modem I would either get a Motorola SB6121 or SB6141
what are you going to use the router for?
Heavy gaming, video streaming, torrents ... That kind of thing theirs a fair number of devices connected to the old one (3 smartphones, 1 PS3, 1Wii, 1 LG Smart TV, 1 LG Smart Blue-ray player, 1 I-pod touch, 2 laptops, 1 desktop, 1 desktop Ethernet connection, and 1 printer.) so it's been crashing pretty frequently. I was planing on using an old Pentium 4 at something like 3.23 GHz with an 500 gig hard drive and an Asus PCE-AC66 Wireless-AC1750 Dual-Band PCI-E Adapter
) with pfSence.