Buying RAM these days is really about sticking to some rules and buying from a reputable manufacturer.
1. Buy DDR3: Only legacy sockets from AMD and Intel support DDR2. If you're building a new PC, you're going to be buying a motherboard that supports DDR3, and DDR4 support is likely a year or more away.
2. There are only 2 RAM speeds worth worrying about: Unless you're willing to spend lots of money for extremely overclockable RAM, there are only 2 DDR3 speeds you should be thinking about: 1333Mhz and 1600Mhz. In real-world situations, there isn't likely to be any significant difference between performance at those two speeds. More expensive, faster RAM may make your benchmarks look good, but in the real world it doesn't provide enough genuine performance benefit to be worth the price premium. Similarly, buying desktop memory at sub-1333Mhz speeds doesn't save you any significant amount in comparison to the performance detriment (if you can even find memory that slow).
3. Don't worry about timings: Tighter timings (lower timing numbers) mean that your RAM is faster, but generally speaking you won't see significant real-world benefits from tighter-than-standard timings. Standard 1600Mhz RAM usually runs at 9-9-9-24, which is just fine. Just like RAM speed, generally speaking tighter timings aren't worth the price premium associated with them.
4. Buy at least 4GB of RAM: Memory utilization for Windows 7 is actually pretty good, but memory usage for programs in general has scaled up over the past few years, and you're definitely going to want at least 4GB of RAM, 8GB if budget allows.
5. Buy matching DIMMs: Try to buy your RAM in kits (sets of matching DIMMs). If you can't (maybe you already have a single DIMM in the system), you should match model numbers (not just "2GB Corsair Vengeance" but the actual model number of the RAM, "CMZ8GX3M2A1600C9"). So if you're running a single 2GB DIMM and you want to add a second, make sure you purchase an identical 2GB DIMM from the same manufacturer. It's entirely possible that non-matching RAM will function, but you'll eliminate a lot of potential conflicts, issues, and headaches by ensuring that your sets match.
6. Buy the right kits for your multi-channel setup: So if you have a Triple Channel board (only boards using Intel's LGA1366 socket, like the X58 platform), buy in sets of 3 DIMMs. If you're running on a Dual Channel board (all current-gen AMD chipsets and all non-LGA1366 Intel chipsets), buy in sets of 2. If you don't fill the channel, you're sacrificing performance by not taking advantage of the multi-channel setup.
7. Follow guideline #6, but always buy the fewest DIMMs possible while doing so: So if you're on a dual channel board and want 8GB of RAM, don't buy 4x2GB sticks. Yes, you're buying in sets of 2, but you'll be filling all your DIMM slots and inhibiting future upgrades. Buy 2x4GB sticks instead.
8. Pay attention to clearance: Keep your CPU cooler and DIMM slot positioning in mind. Many of the large aftermarket tower style coolers overhang one or more DIMM slots (or have a fan that, when mounted, does so). Avoid contacting your RAM heatsinks with your CPU heatsink or fan, even if no pressure is applied (pressure can result in long-term damage to the DIMM and/or DIMM slot).
Some solid manufacturers: