to maximize system performance. Running 3 DIMMs or 4 that are not matched by capacity forces the system to 64-bit (single channel) vice 128-bit (dual channel) operation. Most mfgs recommend RAM from same brand/mfg, same capacity ...1GB/2GB/4G and same speed.
At this point ...you can try it and see how it runs. Depending on the programs you run...it may be fine as is.
PS...If your system has 4 DIMM slots ....it's DUAL CHANNEL, not triple channel architecture.
I own an Acer desktop running Windows 7 home, it came with 3 GB of RAM, a single 1 GB stick along with a single 2 GB stick. The MoBo has a total of 4 slots though, so a few months ago I ordered 2 more 2 GB sticks to boost the total RAM up to 7.
I remember seeing on another thread though that ram can be used in "dual channel mode" or "triple channel mode", if the RAM is dividable by 3 or just by 2. And this got me to wondering... 7 is a prime number, not dividable by anything. Would my computer run faster/smoother/better if I remove the 1 GB stick?