Thats one point to mention, don't boot without the memory as it won't boot anyway.
Even though its booting up, there might be a problem somewhere with one of the wires, the ones with the hard drive indicators, etc which might stop it from starting the boot process, even though fans are spinning up.
Also check for a 12v slot on the motherboard, since if this is sometimes not connected, it will boot but some things won't, for example, the monitor. I had this. It should be a spare wire coming off the pSU that should connect to something on the motherboard close to that long ribbon power cable.
Plus make sure your not connecting the monitor to an onboard video slot, if you have one that runs off PCI or AGP for example (I'm not saying you wouldn't of checked this, but tis just an example) since this wouldn't make the monitor work either.
Hope some of these suggestions help