It entirely depends on what you want to render. Currently, hair is very slow on GPU and many features are not supported, e.g. volumes and SSS. If you plan on rendering things other than that, GPU rendering is a whole different level of work and you absolutely should buy a card. It makes everything easier.
Which card you should buy also depends on how large your scenes will be. The new 750Ti is getting great reviews, and you can buy two for $300 and reach performance on par with a $1,000 Titan. But it only has 2GB of memory and big scenes with lots of polys will fill those 2GB up fast. If you think that you will keep under the 2GB limit, the 750Ti is an ideal choice that does not require a new power supply, usually even with dual cards.
If you are willing to move up the price ladder, you can future-proof yourself with the 6GB GTX 780. 6GB is a good amount of space and very few people will fill that up. If you buy an overclocked version such as the STRIX from Asus, its performance is very similar to a 780Ti, but with 6GB of memory. Sadly, Nvidia is still trying to convince people that the Titan isn’t a rip-off and is thus refusing to allow OEMs to create a 6GB variant of the 780Ti.
If I were in your shoes, and I didn’t need the memory, I would buy dual 750Ti cards. But since I’m always paranoid about buying something only to have it deprecated the next day, I would wait a couple of months to see where the prices drop and grab a 6GB 780. Then, in the future, when prices have dropped a even more and you’re desirous of increased speed, buy a second 6GB 780.