my Ram will be more $$$ because the P35-DS3L takes DDR 1066. I'll have to get 2 gigs instead of 4. how much will my performance suffer by getting 1 2gb instead of 2 1GB so that I can more easily upgrade to 2x2 later?
No no! DDR2-1066 means it can support UP TO that speed! You could run DDR2-533 for all it cares and run fine! Stick with DDR2-800. Anything over that is useless.
1x 2GB stick will lose some performance due to no Dual Channel...