If your power supply can handle it, yes, it would be similar to what i did some years ago, i hand a pentium dual core pc and what i did is that i bought a new psu first, then a gpu, a HD5870, then i changed the motherboard, cpu, RAM, and case. In games that use more cpu, you will feel the slow down, but you' ll know that just one more upgrade needs to be done, i was in the same position and i did not regret it, but it was me, and games in that time used less cpu so i did not feel it that much. What gpu do you have right now?? and psu??