I'm not the expert when it comes to the technical reasons and calculations regarding how much memory you actually need. But I'm actually thinking right now about increasing from 4GB of DDR2 to 8GB ...and the prices have shot up about 75% in the past four months.
The 2x2GB Corsair kit I bought when I built my system was about $50 in Oct and now is $87 at a popular online site. I'm contemplating the same thing you are, but I think the difference is that I MIGHT already have enough memory for the near future. I'm not sure you do? I've watched memory requirements steadily rise for an awfully long time ...and I doubt DDR2 is going to be any cheaper a year from now. But that's just my two cents...
Wanted to add a bit and clarify that my situation may be different than some. I just built a bit of a decent economy desktop system and want to keep it viable for as long as possible. I also have 64 bit Windows. If you don't plan to keep your current system all that much longer and don't plan to ever upgrade it to 64 bit, then the 2GB might be fine! I only run 2GB on my Pentium D desktop at work and it seems to put up with most everything I throw at it.
OK, wait a minute, I thought you were talking about DDR2 (see thread subject )
...that makes a world of difference! The prices of DDR3 probably will come down some more. Completely different situation than DDR2. My Mobo only supports DDR2 and that technology is going to get more expensive as it becomes more obsolete. DDR3 is becoming the norm, so it stands to reason that prices will drop some over the next year or so.
OK, putting away the keyboard and hitting the sack!