You could use Skype on the two PCs. Skype phones are quite cheap (relatively speaking) - but all the ones I've seen still need to plug into a PC, so you may as well use the headset etc on the computer.
You can get Skype phones that don't need a PC - they plug straight into a router. I think there are some that can work with a WiFi network, again without a PC involved. Yup, they cost a few quid but you may want to offset the set up costs against long-term cost savings.
I've got a great little Netgear set up - the base station plugs into both router and landline and the wireless (DECT) handset offers the choice of Skype or landline. Of course, with the 'pay for' Skype - can't remember what it's called - you get to make cheaper calls to landlines....makes a big difference if you call overseas a lot. Payback of the cost of the kit was about three months for me - almost never use the landline now except for local calls.