If anyone's interested, I have now found out what can be upgraded on the Alienware X51.
-You can use any processor that utilizes the lga 1155 socket, eg: sandy bridge / ivy bridge lineup.
-Your gpu can be upgraded up to a gtx 670, or gtx 660ti with modest factory overclock.
-Memory can be upgraded up to 16gb however you should insure that it is rated for plug and play, otherwise you will not see the advertised speed of 1600mhz. (an example is kingston pnp ram) Note: Just like with the processor you cannot manually overclock ram in the bios.
-Hard drives can be upgraded to either one 3.5inch or two 2.5 inch drives with adapter cage. You can also fit a 2.5inch ssd over the top of the optical drive, either by zip tying it in place or by simply laying the x51 in the horizontal position.
-Last but not least, optical drives. Any 12.7mm slot load slimline drive will work as long as it uses a sata connection.
With all of that said, You will want the 330w PSU. This part is only available from dell so you will have to call them and order it BEFORE you buy anything else.
I'm pretty sure this is the same PSU used by the m18x laptop.
Mogbat