I found two my old GPU's (AMD Radeon 4850 0.5GB and 6870 1GB), what kind of hashrate I would expect from these cards? And if I connect 2 GPU's to mobo, do I need the SLI bridge adapter for mining?
It simply won't work at all. You need at least 4 Gb VRAM to mine ETC and 6 Gb VRAM for ETH...
Not quite correct
for ETC you need 2.6GB VRAM currently
for ETH you need 4.1GB VRAM currently
Yes indeed.
But there not point to buy a 3Gb VRAM to mine ETC unless you already have the VGA and with a 4Gb VGA you can't mine ETH and there no 5 Gb VRAM VGA...