Same here. Every mobo I've bought that supports SLI came with a SLI bridge so yeah I don't think your mobo supports it. Unless you can do a mod like some old nforce 4 boards, you're going to need a new mobo.
Amazon has the MSI 680 http://www.amazon.com/gp/offer-listing/B007V9JL1C/ref=sr_1_7_olp?ie=UTF8&qid=1366207517&sr=8-7&keywords=gtx+680&condition=used for under 400 but those are used. Some stock 670s are around 350 though so you can save some money going that route. Although the fans are louder...
I think geforce gt that are 630 or lower will fit your budget. Maybe something like this: http://www.amazon.com/PNY-NVIDIA-GeForce-PCI-Express-VCGGT620XPB/dp/B00847LDTI/ref=sr_1_24?ie=UTF8&qid=1366148331&sr=8-24&keywords=geforce+gt
Only problem I had with IC Diamond is that its was super hard to spread out easily. In hindsight I should have just squeezed a pea sized amount and let the pressure from the hs mounts spread it.