Old Gaming System As a Miner?

I know this is not ideal or practical in any way, but it's something I've wanted to try as I've never really done any mining of any cryptocurrencies. I have an old gaming system I built some time ago, and as an experiment I'm considering on setting it up as a mining rig. 

Quick Specs

• AMD Phenom Processor @2.4GHZ (Quad Core)
• 4GB DDR2 RAM @ 800MHZ
• 2X BFG GeForce OC MAXCORE @ 896MB /Each (SLI)
• 1TB SATA 2 Seagate HDD @ 7200RPM

Now the question is. What should I mine? What OS should I use? What software should I use?

Please support my project/experiment by leaving your comments/sugegestions below!