Normally the PSU will be able to feed 2 5830.

One of my rigs consists of 4 HD7790 oc'ed (1200 MHz) GPU's each hashing at 330 - 350 MHash/S. The PSU is a 550 W BeQuiet and the whole system "only" consumes 410 W. The most energy efficiency with modern PSU's are achieved at around 80 - 90 % of it's max capacity.

The maximum board power of a 5830 is 175 W, double that for your two. Than you still have enough space left. Every card can leech 75 W out of the PCI-E slot, so the 12 V PCI-E rails of the PSU will only be charged with 200 W in total... but if you consider 300 MH/s in average for the 5830 with averaging 140 W of power consumption compared to 80 W a HD 7790 would need, imho the most inefficient part of the mining rig, will be the elderly 5830...