What exactly is the problem, and what's the spec of the machine?
For a similar wattage PSU, you couldn't go far wrong with another HX520W to be honest. If you fancy a change, the Zalman ZM500-HP is supposed to be good, as are the Enermax Modu82+ and the BeQuiet! Dark Power Pro.
All these are well-regarded for their low-noise.
Can't run 3D apps! Artifact nightmares!
E6420 C2D
Gigabyte GA-P45-DS3
4GB OcUK Value RAM
Powercolor 4850 1GB PCS - On RMA with OCUK
*hint hint, can I have it back please... 
*
Creative Xfi Gamer - Currently out of system, thought it might have been Creatives shoddy Drivers.
2 x HDD's 1x 500GB and a 160GB I think. Can't remember brand. Who cares?
Some Lite-On DVD-RW thing.
Stock Intel HSF
I've an old 7900GTO in at the minute, but thats artifacting too. Although I think thats why I replaced it with the 4850 in the first place. Besides its been sitting on the floor of my room unboxed in a corner for a month and a half, so I figure I left it there because it was broke. Still, it lets me boot Windows.
Will be adding another couple of HDD's once I get rid of the error. Have replaced
everything (except PSU) in the PC with alternative components to test, and tested in other PC.
Will be chucking in a TRUE HSF with the PSU order and probably 4GB of Branded RAM, probably Corsair, the OCUK seems fine, runs Memtest and works in another PC flawlessly, but you never know, motherboard might be picky about it.
Going for the higher wattage of ~700w just to leave room for further upgrades, that's why I'm not getting another 520w HX. Pondering just going for the TX 650w, but the Hiper reviews better thats all...and then it occurred to me that a quieter one might be an idea.
The 4850 RMA might very well fix matters, but I need to be working on the PC, and cant have it out of operation much longer. So, I reckon its best if I just cover all avenues, rather than wait, find the 4850 wasnt the problem and then have to wait on replacement bits coming. Again.
Can't go with the Zalman. It has Blue LED's. Inside of case is lit red.
But not passively cooled. No way. Wouldn't touch that with a bargepole.