If you're building a PC from scratch, then you can easily upgrade anything any time you like. However, if you're asking this question then I think it's a safe bet that you're not building your own PC.
Without knowing anything about what you plan to do on this PC (if all you're doing is surfing "pron" then both options are serious overkill

) I'd opt for the quad core PC. Memory is easy to add later if you need more. I'd also ask if you really need a high end graphics card. If not, I'd go for CPU first and then 4GB if I could afford it with the savings from the video card. Like memory, you can always upgrade the video card in the future if you need to.
For what it's worth, my home PC is a quad core with 2GB RAM and, so far, the audio and video processing I've done on it have been problem free. I've haven't seen it break a sweat yet. Photo processing is even less taxing on it.