You can get a few trim pots to act as variable resistors. You don't have to connect all 3 contacts. I get them 10 for US$1 which by now is about 25 cents Canadian?
I've got old computers at my parents place. I've definitely used the powersupplies before for +12v (jumper the green mobo wire to ground). Is the +5v rail suitable to power the Arduino unit?