Frankly I'm running a GTX 260 with my E8500 which is a little more powerful than both those cards and I am still getting full GPU utilization during games so it's probably a random guy with no knowledge giving that comment
And no, unless you have two identical cards (either two 7750s or two 450 cards with an SLI/Crossfire bridge though the 7750 doesn't need a bridge) that configuration will not work
It seems that your board only supports crossfire, though you won't really see too big of a performance benefit (probably 60-70% more or so) if you used two 7750s with a stock E8500
Also, the secondary PCI-E slot operates at a slower speed so I wouldn't recommend using that slot
So basically, you can only use one of those cards at any given time