LOTS of misinformation here.
1. Command rate needing to be set to 2T for 4 DIMMs is an issue with the Athlon 64 memory controller. We do not know if the OP has an Athlon or a Core 2. All a 2T command rate does is posts the command to the modules for 2 cycles rather than 1, and is much less of a limitation than the horrible latencies in 2GB modules anyway.
2. While 2x1GB is better than 4x512MB, the same CANNOT be said about 2x2GB vs 4x1GB. While 1GB DDR2 DIMMs have been out for a long time and they are now cheap and have nice speeds and timings, 2GB modules usually cap at 667MHz (there are a very small selection that will run at 800MHz for $$$), and have horrible latencies.
5-5-5-15 @ 667MHz is not uncommon for 2GB modules, while 1GB modules come in 3-3-3-12 @ 667MHz and 4-4-4-12 @ 800MHz.
3. 2GB modules are significantly more expensive in terms of $/GB.
Basically, unless you can forsee a future need for 6GB or 8GB, and by the time games need that you'll be using DDR3 anyway, go with the 4x1GB.