Birthday Pc Parts Dilemma Input Wanted

Today my wife told me that she has set aside $80usd for me to buy a new part for my PC. I have narrowed it down to two parts but cannot decide which one I want. Corsair Cooling Hydro Series H50 High-Performance CPU Cooler or WESTERN DIGITAL Caviar Black 640GB SATA II 32MB Hard Drive

I'm currently using the stock AMD hs/fan and have one WD caviar black 640gb hdd. The cooling would be a big improvement, but on the other hand I would have two nice hdds in raid 0.

What would you do?
------------------------------------------------------System specs:
CPU: AMD Phenom II 955 BE
M/B: MSI 790FX-GD70
RAM: OCZ Obsidian 4GB 2x2 @1600MHz
VIDEO: ATI/Sapphire HD 5770
PSU: Corsair HX620W
HDD: WesternDigital caviar black 640GB
CASE: NZXT Beta EVO