Time once again to learn the in's and out's of memory which right now is SDRAM DDR3, but the basic facts are that you can overclock memory a little and in most benchmarks, once you go from vanilla DDR3-1333 to DDR3-2133, you have had a huge improvement in performance at a relatively low cost. If you really want to splurge, then go to DDR3-2600 from G.Skill and you will be in Ferrari heaven 🙂 This is in effect all about the pricing curve. The memory choices are DDR3-2133 up to -2666.
Unlike years past, xBitLabs says that for the last few years seems that processor speed makes less difference and memory makes less difference to gaming speeds. It seems to be all about the graphics cards now and to a lesser extent the disk storage system. So most folks use DDR3-1333 or -1600 and they are just fine.
Ivy Bridge removes the top speed limitations, so it could theoretically go to DDR3-3200 (3.2GHz) which doesn't exist yet and you can get memory at DDR-2400 which would be twice as fast as standard ram. Wow. They did a nice testing run using the i5-3570K and the Sandy Bridge equivalent i5-2550K (for Intel geeks, the model number encoding is i5 means no hyperthreading and slow on board graphics than i7, the 2 means 2nd generation of Sandy Bridge, while the 3 means Ivy Bridge, the last digits are the model number and K means unlocked, got that???)
In their review, they used the highest rate they could find which was the G.Skill TridentX F3-2600C10D-8GTXD which is an 8GB kit running at DDR-2600. Interesting to see that you can't really overclock the memory, it can run only slightly faster to 2.7GHz, while the CPU norminally at 3.8GHz, could overclock to 4.5GHz! But they recommend going with the more affordable DDR3-2133 and you can see 5-20% increases in memory intensive applications like games. Also in the old days things like CL2 or CL3 or intermediate timings made a huge difference, but with today's memory the most important thing is raw clock speed.
According to Tom's, the current state of the art is quad channel DDR3 (whatever that means :-). The review shows that for Sandy Bridge, but what about for Ivy Bridge, xBit did a review of these and found

• The Mushkin Redline 993997 has 98% of the overclock performance for just $210 for DDR3-2133 memory. Newegg has this for$129
• The highest performing 16GB RAM kit was the G.Skill Ripjaws ZF3-2400-16GBZHD running at 2400 GHz with
• Quad-channel kit consists of four memory modules, 4 GB each;
• Nominal frequency: 2400 MHz;
• Timings: 10-11-10-31-2T;
• Voltage: 1.65 V.

Newegg didn't have this model but at $159 which was model F3-2400C10Q-16GZH which is • Cas Latency: 10 • Voltage: 1.65V • Multi-channel Kit: Quad Channel Kit • Timing: 10-12-12-31 • There is also at 16GB, the G.Skill Trident X which runs at a remarkable DDR3-2600 F3-2600C10Q-16GTXD for$329 at Newegg

The net is that with this high end memory, once you get to 2133 for $129 is an easy tradeoff and to get an addition 2-3% performance, going to 2400 for$159 is a decent deal.