It really depends on how far you're going to go with it, the type of work you plan to do, and how many pixels you plan to drive. I would argue for simple parts and small to medium assemblies in Inventor, you don't need any graphics card at all. Once you get into moderately complex assemblies, or if you plan to do any CAM simulation in Inventor HSM, then a graphics card is a good idea.
We run Quadro K620s with a single monitor at 2560x1440 and we're pretty happy with the performance there. Only slight hiccups in HSM CAM simulation of advanced paths, but buttery smooth in the CAD environment. You may want to go to a slightly higher spec if you plan on running 4K.
I run a Quadro K2200 across three displays at 1920x1080 and again, buttery smooth, and a bit more smooth in HSM CAM simulations than the Quadro K620.
The Quadro K1200 and M2000 are also out now, with the M2000 being about the best card within your budget, by my guess is that's pretty overkill for whatever you're going to be doing. Most people who do this for a living are not even running that high spec of a card. But hey, I don't know, maybe you have a need to do CAD on four 4k monitors at the same time....
My advice, save a little on the graphics card, and use whatever you save to go for a nicer monitor. Working at 2560x1440 on a 27 inch is nice. I run a Dell U2715H that I'm happy with except for the issue where it doesn't wake on a mouse jiggle with Windows 7. The HP Z27 series monitors are also nice.
You may even consider a 34" 3440x1440 Ultrawide. The prices are down to about $500.
Finally, Inventor does work well with gaming-oriented cards, but the drivers are not tested/certified, and I have no first-hand knowledge of it.
Here is a list of certified graphics hardware for Autodesk Inventor Professional 2017. AMD FirePro cards are certified and recommended as well and can be more affordable. They worked well when we used to use them, though nVidia is probably a bit more industry standard.