Right now I've got a decent gaming rig, I love gaming and really don't want to give it up for a specific GPU. Is there a card out there that is excellent at doing 3D modeling and rendering and playing games? So far I'm using a GTX 670.
it all depends on the program you're using, some will only be open cl supported (amd), some only open gl (both are pretty good), some only cuda (nvidia), and then some don't support gpu acceleration at all
I switch between Softimage, Max, and Photoshop a lot.
A GTX670 is fine for those uses.
If you end up with a big project for instance where a Titan would be useful for rendering in a specific renderer, and it'd justify it's price to enable you to land that contract then go for it.
Chances are though you might make more profit by farming the job out these days unless you are sure to get lots of jobs that can use Open CL rendering or a GPU rendering workflow.
With regard to UI speed, PS should be fine with anything like the card you have. I've been working with 20,000px tall and wide textures here in PS and then in my 3D app, loaded in both at the same time. No problem even with my old GTX275.
And in the end you optimise your 3D viewports. Right here I have 99% of my parts hidden using layers and other techniques while editing things. You don't need huge GPU power really. It's nice to have a lot, but a GTX670 is probably gonna feel just as fast as a Titan in most 3D apps 95% of the time.
Unless you are gonna be making your graphics work pay your bills and be put to use day in and day out then just stick to a good gaming card.
I've worked in computers for creative/technical work for about 12 years now, and also done hobby work along those lines for 16 years and I've had just as much productivity with my high end gaming GPU's as I have with expensive Quadro GPU's or Xeons vs my high end normal CPU's etc.
You'd be surprised how far you can go in the world of serious production for games/video etc and find people just using good normal kit rather than silly expensive stuff.
There is a time and need for that stuff otherwise it wouldn't exist, but it's usually found in the kinds of places where you are working with many many other artists on large scale projects where every second counts in the delivery of huge projects with huge budgets.
Ie, 10% render speed benefit will net the business £200,000 more profit = justification for £50,000 extra spend on xeons and quadros.
Assuming you don't already have it, independent drives for scratch discs/swap files and oodles of ram will be far more useful to you I think.