Quote Originally Posted by '-silencer-',index.php?page=Thread&postID=84716#post84716
64-bit XP will recognize 4GB. 32-bit XP (by far the more common version) will only recognize ~3.2-3.5GB.
Memory performance is best with paired sticks in dual-channel mode (2x 1GB or 2x 2GB with the same speed/latency, preferably the same brand/model).

Just get 2x 2GB sticks, especially if they're at a discount.
I second (or third) this.