Medium answer: What's your motherboard? Those numbers smell like you might have the option of going three-channel, which is a little better than a memory increase.
Long answer: As always, it depends. It depends on the motherboard, it depends on what applications are important to you, it depends on whether or not you overclock, it depends on whether or not Jaquith is going to participate in this thread with charts and illustrations.
You can look at recent benchmarks on Tom's and see the effects of faster memory and triple-channel vs. dual-channel. My personal takeaway from all of this is that the effects are significant but small, so I'm still running a dual-channel i5.
Really short correct answer: 6GB if RAM is compatible and both the OS & CPU are 64-bit, and if X58/1366 use Tri Channel kit of 6GB, other MOBOs Dual Channel, 4GB, is preferable but some per QVL/Tested Dual Channel MOBOs will run 3 sticks but not in Dual Channel mode. In either case verify compatibly via QVL, RAM Mfg's Tested, or per Technical Ticket.
...it depends on whether or not Jaquith is going to participate in this thread with charts and illustrations.
lol good one!
Assuming a 64-bit OS, more RAM is pretty much always better as it allows you to run more programs simultaneously, and makes for a smoother computing experience.
If you have a triple-channel mainboard: 6GB minimum. That would be a 3x2GB kit. The next step up would be 12GB -- two 3x2GB kits or one 3x4GB kit.
If you have a dual-channel mainboard: 4GB minimum, 6-8GB would be better. You can do 6GB on a dual-channel board by having one 2x2GB kit and one 2x1GB kit. I have 12GB of DDR3-1600 in my dual-channel Sandy Bridge system -- one 2x4GB kit and one 2x2GB kit.