can linux tell me how many open memory slots i have

yes it can:
sudo lshw

lshw is a small tool to extract detailed information on the hardware configuration of the machine. It can report exact memory configuration, firmware version, mainboard configuration, CPU version and speed, cache configuration, bus speed, etc.

So if you're running linux and you need to know your graphics chip set, your motherboard manufacturer or model number... but you're too lazy to open the case and look ;)

Try the Linux utility List Hardware.