What is the highest ios update you can get on an iPhone 3GS? (not 3G!)?

What is the highest ios update you can get on an iPhone 3GS? (not 3G!)?

I believe IOS 6.1, the same as iphone4. I have heard rumours that on the 3gs it can make it very slow to respond so be careful if you plan to update, I always leave it a month or so after apple release any new IOS, Then keep an eye on the apple forums and you can see all the issues people are having with it. After a month or just over they tend to work the problems out and release a fix. To be safe, always check the apple web site.
Hope this helps