You will need:
http://www.newegg.com/Product/Product.aspx?Item=N82E16812200214
http://www.newegg.com/Product/Product.aspx?Item=N82E16812123107
+
SSD and also X51 drivers which can be found on the Dell website or the Disks that came with the x51
Here's how
It's actually very easy to open up the case. Just unscrew the back of the x51 and slide the left panel out. After that, unscrew 2 screws to lift up the gtx555. Unscrew 1 more to slide out the dvd drive.
Take off the current 4 pin molex power and connect it to the cable you bought. After that, connect the cable to the motherboard. Connect the sata power to ssd.
Before you put everything back, unplug the HDD to make sure the OS installation on the SSD will have no problems. Keep in mind that this is using the SSD as a boot drive and to hold a couple of other programs. Keep ALL bulky files and storage on the HDD.
After the OS installation, plug back in the HDD and install drivers.
Here's how to enable trim for your SSD (Credits go to whoever wrote it.)
Here's where to mount(tape) your SSD.
Other pics.
And thats it!
http://www.newegg.com/Product/Product.aspx?Item=N82E16812200214
http://www.newegg.com/Product/Product.aspx?Item=N82E16812123107
+
SSD and also X51 drivers which can be found on the Dell website or the Disks that came with the x51
Here's how

It's actually very easy to open up the case. Just unscrew the back of the x51 and slide the left panel out. After that, unscrew 2 screws to lift up the gtx555. Unscrew 1 more to slide out the dvd drive.
Take off the current 4 pin molex power and connect it to the cable you bought. After that, connect the cable to the motherboard. Connect the sata power to ssd.

Before you put everything back, unplug the HDD to make sure the OS installation on the SSD will have no problems. Keep in mind that this is using the SSD as a boot drive and to hold a couple of other programs. Keep ALL bulky files and storage on the HDD.
After the OS installation, plug back in the HDD and install drivers.
Here's how to enable trim for your SSD (Credits go to whoever wrote it.)
Code:
How to enable TRIM Command in Windows 7 with a Solid State Drive
To enable or disable TRIM Command, you will need to open an Elevated Command Prompt window.
How To Open an Elevated Command Prompt window: Click on Start Orb > Type "CMD.exe" in Search box > Right click on "CMD" and select "Run as Administrator" (If you receive a prompt confirmation, click YES)
How to Enable TRIM Command
In the Elevated command Prompt windows, type the following:
fsutil behavior set disabledeletenotify 0
How to Disable TRIM Command
In the Elevated command Prompt windows, type the following:
fsutil behavior set disabledeletenotify 1
How do I know if TRIM is working in Windows 7?
In the Elevated command Prompt windows, type the following:
fsutil behavior query disabledeletenotify
Results explained below:
DisableDeleteNotify = 1 (Windows TRIM commands are disabled)
DisableDeleteNotify = 0 (Windows TRIM commands are enabled)
How to enable AHCI in Windows 7 after Instalation:
There is one way to fix this, although you need to have knowledge of registry editing. The detailed steps from Microsoft are as follows:
1.Exit all Windows-based programs.
2.Click Start, type regedit in the Start Search box, and then press ENTER.
3.If you receive the User Account Control dialog box, click Continue.
4.Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINESystemCurrentControlSetServicesMsahci
5.In the right pane, right-click Start in the Name column, and then click Modify.
6.In the Value data box, type 0, and then click OK.
7.On the File menu, click Exit to close Registry Editor.
After this you’ll have to restart your computer, go to BIOS and enable AHCI. When you log in to Windows again, you’ll notice the installation of drivers for AHCI. Another restart will be required to finish the driver installation.
These steps are provided at your own risk and not suggested unless you know and understand the risks. You will want to make sure the controller driver and MB BIOS support this option before you enable it
Here's where to mount(tape) your SSD.

Other pics.

And thats it!