As long as your PCI-E are +12 and 4pin connector line, you are fine. There is no need to go buy another PSU.
Use 3 cables into one side and 2 cables to the other. It will run! (Trust me, I tested 5-cabel configuration and it mined at 180GH/S for hours)
The PSU I have handy is a thermaltake puperpower 500 (
http://www.thermaltakeusa.com/ProductHtml/W0100RU__Feature.htm)
It has a 6 pins pci-e plug and a 4 pin 12V connector which makes 5 yellow and 5 black, what should I use for the 6th cables? Could the sata/ide cables help there? I can probably use a second 350W PSU as complement, but if there is a (safe) way to do it only with the 500 one that'd be great.
Thanks to sushi for providing the review, I am not sure I would have bought without it.