It has to be a Dell PSU. Unless you are up for modding, which is unnecessary, you only need to get the Dell 330w PSU charger for the X51/m18x/18.
Typing in X51 330w PSU into amazon or ebay should be enough to fiund what you need. As well, Dell sells the 330w PSU.
I hope you realized this before, but if you haven't, the X51 only uses external PSUs. Never has Dell/Alienware sold an X51 with an internal PSU. Your current 240w PSU is that power brick you see outside the X51.
Currently owned computers:
AW13 R2 | i7-6500U | GTX 960m 4 GB + GA (R9 285 2 GB / RX 480 8 GB) | 16 GB 1600 MHz | 1800p Touchscreen | 2 x 500 GB SATA M.2 | Graphics Amplifier Performance Analysis with 13 R1
Desktop | i7-3770 | GTX 980 4 GB | 16 GB 1600 MHz | 120 GB mSATA + 960 GB SSD + 2 TB HDD | 600w PSU
Formerly owned computers:
HP Pavilion 15-n280us (given away) | Apple Mac Mini 2012 (sold) | m15x (damaged mobo) | 13 R1 (death by unknown) | m14xR2 (Tradied In)