Two Sticks. No question. That allows you to upgrade later more easily and should make it easier to overclock if you do that sort of thing. Usually there's a small performance penalty for going from two to four sticks of memory anyway. The good thing is, when you do decide to go from 4GB to 8GB (or 6GB) the additional memory will be quite useful assuming you're using XP-64 or Vista x64. (Vista x64 loves 4GB)
One other thing... If you're planning on AM2+, don't get the DDR2-667s, get the DDR2-800s or better. It may cost a little more, but I promise you'll be kicking yourself later when you go Phenom and you're stuck with 667mHz memory.
Main/HTPC: 880G Extreme3, 960T -> X6 3.5GHz, 4x4GB, HD6950, 2x120GB.OCZ.Agility3 RAID0, Sharp 55" LCD 1080p, Win7x64 Ult & Mint 13 Cinnamon
"Kill all my demons, and my angels might die too..." -T. Williams