You might as well get a cache if you don't fancy an SSD. Either way it'll be one of the most noticeable differences you can make. It's the "going black" of the computer hardware (alongside 120hz monitors).
From cheapest to most expensive (all have free shipping)...
PC P&C Silencer MK III 500w 80+ Bronze (MIR)
Seasonic M12II 520w 80+ Bronze
Rosewill Capstone 550w 80+ Gold
They're all modular too, so you avoid the cabling mess.