Probably the biggest thing that helped with IV is having a 64-bit machine, which is all the sell now. Outside of that it really does not matter much. Better the specs the better, but IV has a known memory leak so no matter how good the machine is the game will slow down after an hour or so of playing - how fast it slows down depends on your specs.
If the game is slowing down for you after a while, well that is normal. Easiest thing to do his hit ctl-shift-ESC. It brings up task manager and appears to refresh memory somehow (learned that from Dhoomstriker). It really works and IV will play faster for some time before you need to do it again.
If your computer itself is slowing down, then this may be an indicator of something wrong with your machine. You probably need to clean up some things, uninstall some crap, and check for viruses/malware. Try Ccleaner and Malwarebytes.
If you want an new machine, any of the game ready desktop/laptops sold nowadays are way more than adequate to run IV, and you don't need to spend a lot. Again, keep in mind that no matter how shiny the machine is, IV will slow down.
(version of IV should be irrelevant)