1| welcome to the forums, newcommer!
do you know if there are any other PSU's like that that are around 500w?
yeah - check out newegg.com and do a search.
3| Stick to brand manufactured PSU's, they are quality makes and have a good tolerance over time. f you want to get a cheap PSU, be my guest, but remember we didn't support getting the cheap psu to create the 4th of july in your rig (and your wallet)
Want some makers narrowed down?
all of them make 550W PSU's. Look out for the xfx 550W Core edition full wired PSU
Edit: just took a look at your rig - go for a this 450W unit http://www.newegg.com/Product/Product.aspx?Item=N82E16817207012
or this 550W unit (in case you want to keep your PSU and make a future GPU/CPU/Mobo upgradehttp://www.newegg.com/Product/Product.aspx?Item=N82E16817207013