Which PC to use for pfSense?

So I have three old PCs, which one should I use?

Pentium D @ 2.8GHz, ATI Express 200 Series, 2x512MB DDR2 RAM @ 960MHz, some shitty HDD

P4 but it only shows 1 thread in task manager? Like 512mb of DDR RAM. Not sure of HDD size

Some piece of shit with a Sempron, 512mb DDR, 160GB HDD

Whatever I don't use I will be parting out.

Either will do; the P4 is probably the faster option, but without specifics, I can't be sure. Regardless, they're both overkill.

Yeah I'm pretty sure they are more specced out than any router lol. 

So wait, pfsense is basically taking a PC and making it into a router?

sure is! I'm using a Intel Pentium 4 in my PFSense box with a gig of ram and a 40gb hard drive

pentium D is essetially two physical pentium 4 chips on one Pcb it would be faster, but also a lot warmer, the more you want pfsense to do the more processing power you want, like virus scanners, caching, and other stuff

I have a router already set up (some linksys thingy). What would happen if I turned my old Athlon based PC into a router with my current one already up and running?

It would basically setup another network. Also I would setup OSPF or static routes so your packets of information can be sent between the two networks.