I bought a second hand computer which I bought so I could play SHIII. I add some extra memory and a graphics card (it's an old FX5200, but it's the oly one that could fit a low profile AGP slot)
It did fine with SHIII and coped well with GWX (so far!), it does drop frame rates around port but there is a lot to keep track of!
After seeing the specs for SHIV I know that my computer will be OK to play it on a low setting for the game (it will just make it!), but it will be hard pushed to cope with any upgrade! (looking forward to a Pacific GWX already!), so I'm going to buy a new computer around the middle of the year (as long as I can get permission from the paymaster general, the wife!)
Hope this keeps me going for a couple of years at least!