Yes, 3.0 cards will fit in 2.0 slots. With the Titan, PCI-E 2.0 is about 5% slower. Lower GPU's such as 7970's and 660Ti's will have less of an affect.
Thanks I looking at the EVGA GeForce GTX 650 SUPERCLOCKED 2048MB GDDR5 for a low budget choice. Seems to have good reviews. Any pros or cons with this card?