Yes. A 4x PCI-E 2.0 slot is not enough to get worthwhile performance returns from crossfiring R9 270Xs. I'd recommend an 8x PCI-E 2.0 or an 8x PCI-E 3.0 for the second card, to make it useful.
Even then, I typically don't recommend crossfire or SLI. Just way too many problems to be a good choice for most people.