Brand New PC or Use Some Parts from Current PC?

I am considering building a new gaming PC at the moment and would like some feedback about whether I should get all new parts to build it, or use the power supply and hard drives from my current gaming PC. The advantage to all new parts would mean that my old computer would still be operational, but it would cost a few hundred dollars more. I guess what I'm thinking is that if I am considering upgrading, then what good is an old gaming PC? I guess I feel like I don't want to "waste" it since it is still a decent machine for everything except the newest games. I was thinking I might be able to sell it, but nobody would really want to buy an old gaming PC that would require them to buy an operating system, power supply and hard drive.. they might as well get a console or a new PC for themselves.

Sell your older PC whole and buy new parts,unless you're on a strict budget

It would help if you tell us the specs. I have build around my rig since 08 because I'm cheap like that. Same case, same psu, same heatsink, same barely functional dvd drives, case fans, etc. I save quite a bit that way, but there a drawbacks like my old case still has lame floppy style enclosures beneath the dvd drives and the heatsink doesn't compare to modern ones.

Parting off your old parts like cpu and gpu can add towards your upgrade.

The money saved though maybe able to squeeze an upgrade and new console.

Currently I've got 8GB DDR2 800MHz RAM, Core 2 Duo E6600 2.4GHz overclocked to 3.15GHz (stable), Antec 750W Power supply, Nvidia GTX 460, 1920x1080 monitor (25.5"). I've been able to play most games recently, but I finally quit blaming the netcode in Battlefield 3 and I get 30 FPS max on it. Battlefield 4 goes anywhere from 6 FPS to 25, but mostly stays around 20.

At the very least, I would have to upgrade the motherboard, CPU and RAM, although I have a feeling the GTX 460 is too old too.