Check out the threads in the Administration forum, where the best information on hardware resides. Do a search on "best graphics card" or something like that. You will find a thread on the testing that Charles Culp and Anna Wood did last year. I believe they will be presenting similar information at SolidWorks World next week. The new information should show up in the Administration forum in a few weeks. There is also lots of good information on processor choices as well in other threads, particularly by Charles, in that forum.
Video card is not what is slowing you down. How big of assemblies are you working on. If you do "AssemblyXpert", how big is the assembly?
Yes, your 2.6GHz processors aren't helping anything. You should really be using a Desktop with a 3.5, 3.6, or 3.7 GHz machine. Or if you can, an Xtreme machine from Boxx: http://www.boxxtech.com/Products/3dboxx-4150-xtreme or to really make your problems disappear, this: http://www.boxxtech.com/Products/3dboxx-4920-xtreme
If you would prefer a boutique builder (cheaper than Boxx), @Xi makes this: http://www.xicomputer.com/products/Configure_prof.asp?model=mtowersp4&configid= with the Intel Core i7-4770K at 4.3GHz with the AMD FirePro W5000. (roughtly $2,200)
Whatever you do, make sure you have an SSD boot drive, and use that for all your solidworks working directory. A PDM system helps with this, but you can do it by hand if that's how you are set up.
You could also set your large assembly mode threshold lower. If you are not doing editing in the assembly, just mating parts, you can load the parts as lightweight. This will drastically improve your performance as you will not be loading all the geometry of every part.
- For 2013
Tools - Options - System Options - Assemblies - Check the 'Large Assemblies' box and lower the threashold number. You will need to tweak the number until you find one that works for you and your computer.
Yes we all wish we had the fastest newest computer, but they really aren't needed. I have designed on older computers just fine. It's about using the software to do what you want it to do...not letting it control you. Your setup with an i7, 16GB of RAM and K3000/4000 video cards is pleanty of processing power.