Hi everyone. Just sharing my experience and I did not find much about this so far so here it is:

1) X51 system only has 1 slot for 1 hdd. :( I saw other ppl using a Y adapter power cord and leaving the SSD on top of the DVD/BR drive) Not sure if this is a smart idea since the bottom of the SSD is metal and you placing it on the optical drive's metal top. I just dont want to fry my ssd.
But if you go with this. Make sure the your adapter is a real Y adapter. The one i had was different.The 4 pin connector's 1 side was male and the back of it was female. The only cable was coming out from the double sided connector was the power for the SATA connector. So if you connect that to the motherboard you had to connect the power cord (for the hdd and optical drive) on top of your adapter. That would make the adapter and the power cord really tall and you wouldn't be able to put the optical driver back.

2)If you decide to replace your HDD with an SSD using an adapter watch out because some adapter will make the SSD sit higher. And it will block/seal the fan on the GPU.

3) Watch out for the GOU cables. There is only 1 gpu but there are two cables and while 1 is connected the other 1 could be in the way of the fan. So make sure when you put the gpu back the other cable is far away from the gpu's fan. I had several system shut down, before I realized it.

4) You can keep the inner temp down if you replace the hdd with an ssd and buy an usb3 external case for your hdd.