I own the 8400 gt already from a computer thats powersupply died. Where can I find a used dual core system?
Depends on where you live. Craigslist, ebay, etc...
You may need to hunt around for a good seller though, many people in craigslist are just trying to resell computers and parts that are crashing on them. I've gotten good things, and crap off there. An older model dual-core should run a bit over $100. Just make sure you see it running, and not just that, load up a game or 2 in it for a bit to make sure it stays running.
Everything I sell on craigslist I run though several torture tests and do a disk scan on, probably not a lot on there do that though.