Depending on your system, especially your motherboard and CPU, upgrading to 4GB of system memory could be slower, or just as fast as 2GB. Throughout the article, we are considering 4GB to be (4) 1GB modules, or 4 * 1GB. While 2 * 2GB modules exist, they are scarce, expensive, and slow. 2GB modules are best suited for servers, because most are buffered, which is why they are so expensive.
The performance increase or decrease will depend on whether you have an AMD or Intel. For my benchmarking tests, I will be using 4 sticks of PC2-6400 memory and 4 sticks of PC-3500 memory. Now that AMD AM2 has finally dropped, the DDR2 portion of the performance results may prove most interesting, since both the Intel DDR2 and AMD DDR2 are rather similar. One thing to bare in mind, is that as soon as you go from 2 * 1GB to 4 * 1GB, you are in effect going from 1T to 2T timings, which are latency related. 2T could prove up to 20% slower than 1T, which is why many systems will indeed suffer memory bandwidth/latency slowdowns, especially in synthetic benchmarks.
That’s where 2 * 2GB will be beneficial, because even though it’s still 4GB, it should still able to handle the 1T timings. I won’t get into how 1T and 2T timings work, because that would be an entirely separate article. To put it simply though, 1T is harder on the motherboard and memory controller, so having a 4 * 1GB configuration at 1T timings is far too taxing for any current motherboard and CPU at higher MHz, regardless of how high end it is. If you wanted to decrease the memory speeds so far in order to hit 1T with 4 1GB sticks, you may as well haul your 386 PC out of the closet.
Simply put, if you have an AMD and are going to 4 * 1GB, you will experience slower speeds than you would sticking to a 2 * 1GB configuration. With Intel’s, it could be either way. If you are using an nForce 4 Chipset, meaning you can do SLi on your Intel, then you will be taking the same route as DDR1 on the AMD. The nF4 Intel chipset allows you to use 1T timings for a 2 * 1GB configuration, and 2T for 4 * 1GB, so you will again experience slowdown with the latter config.
If you happen to have an Intel chipset instead, such as the one on my ASUS P5WD2-E, then you will be using 2T regardless of what your configuration is. This is one reason why DDR2 seems slower in some cases than DDR1 when compared, because even a 2 * 1GB config on an Intel chipset uses 2T. Why this is the case, I have no idea. What this does in turn mean though, that moving to 4 * 1GB on the same chipset will keep the 2T timings, so the performance decrease should prove minimal… if any.
Note, that the descriptions above also apply for things that don’t use all a full 2GB.
On an extra note though, even if your PC can handle DDR2 memory at 1T, chances are you will not be able to retain that 1T timing above DDR2-800 speeds. At that point, the strain is becoming increasingly painful on your memory controller. Therefore, for either a 2 * 1GB or 4 * 1GB configuration with faster than DDR2-800 speeds, the performance should not change at all.
As mentioned earlier, this is where people will actually benefit from having (2) 2GB modules, because they will retain 1T timings. The primary reason why this is not good for the average consumer, is because they are slower. I believe they max out at PC3200 speeds, and many come in ECC flavor, but not all. This means you could expect to pay an upwards of $800US for an low speed 4GB kit.
Without a doubt though, engineers are working hard to produce chips for 2GB modules that don’t suffer from a huge lack of speed that the current ones do. Ryan Peterson, CEO of OCZ Technology, was recently quoted as saying that they would be releasing 4GB kits consisting of 2 * 2GB, later in the year. The expected launch will be shortly before Vista, because that OS will prove to be system intensive. Considering Vista can take an upwards of 500MB of your ram on a fresh boot, that leaves you 1.5GB for your other app’s and a high end game. Not exactly that much breathing room, depending on what you are trying to do.
As soon at these 4GB kits are available, chances are that they will be superior to the speeds we are seeing from current 4GB kits. I am doubtful we will see the ultra high-end speeds that we do right now, but I believe 2 * 2GB kits with PC4000 and PC2-6400 speeds should be very possible.