I would suggest the i5. Other factors that affect your multitasking ability are ram quantity and to a lesser extent hard drive speed. Smooth multitasking takes at least 8gb of ram, if heavily multitasking then going to 16gb may not be a bad idea. It really depends on your use. Having tabs open in the browser isn't overly cpu intensive, more memory intensive. Sometimes things are actually multitasking such as encoding a video in the background while you're playing a game and running a virus scan (not generally recommended since it will impact performance running scans which can be rescheduled for other times).
Sometimes people think they're multitasking when in fact they're doing a lot of single tasking. Having say notepad open jotting down info, the calculator open because you're sorting some numbers, the browser open with 30 tabs and playing a game. No one is literally typing down notes AND playing the game AND surfing the web, they'd need 2-3 keyboards and multiple mice. Instead what's happening is you're quickly switching from one task to another and doing them individually, one then the other, then the other and so on. This is where memory plays a key role, if you've got plenty of memory to keep needed working data in system memory it can be called on much faster than if you run low on ram and it dumps some of the data from say the browser out of memory to make room for the game. You switch back to the browser, now it has to search the hard drive for the data that was previously in memory. That will cause 'multitasking' slowdowns.
Many times a cpu will hop from one task to another to another and give the impression it's doing all those things at once because of how fast it's capable of switching gears from one program to another. Despite having 'only' four cores, intel's chips have incredibly strong core performance and processing power and are easily able to handle quite a bit. A 4590 will likely keep up with an fx 8350 in multitasking without problem and do most other tasks faster. Just my .02 worth.
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