I'm currently running Inventor and Solidworks on a 6700k and GTX 1070. Like Barin said, only saying i5 or i7 is almost meaningless without the full number. The graphics cards your currently running are super ancient in 2018 as Intel's modern integrated graphics solutions on even the low-end chips like Pentiums will blow the 8400GS out of the water.
If you are only interested in modeling and assembly workflows single threaded performance is the most important factor as SolidWorks and Inventor almost exclusively use single threads in those workflows. A modern core i7 or i5 will do absolutely fine in that case. If you're interested in simulation and rendering, on the other hand, looking for higher core count solutions like an AMD Ryzen processor which are similarly priced would be recommended.
As far as the graphics cards are concerned, my GTX 1070 has never gone above 15% usage in Inventor and the Quadro lineup is significantly more expensive than the GTX series for the same specs. The GTX series is going to be significantly cheaper and work great, CAD programs will tell you that gaming cards are unsupported but they work just fine. Integrated graphics from Intel are also very usable but a dedicated card is almost always going to be better. I would also recommend staying away from all AMD GPUs for your case.
The more ram you can get the better as well. I am currently running 32GB and have probably used a maximum of 18GB while working. I'd probably recommend using a 16GB kit of DDR4 for a workstation or more if you can get it, but ram prices these days have been rising. If your running on a Hard Disk Drive (HDD) for your storage upgrading to an SSD (I especially recommend the M.2 form factor) will also save you a lot of loading time and they are pretty reasonably priced these days.
What specific components I would recommend would depend upon the budget, but a modern i7 like the 8700k with some ram and a decent drive will run almost anything you want great these days. Puget Systems' website has some pretty great analysis of different hardware running on Inventor and Solidworks and is a good reference. PM me here or on the Vex Discord if you want some more specific recommendations.