Im pretty sure you mobo doesnt use dual channel, so it doesnt matter which slots you use.
Your DDR400(PC3200) Ram will underclock itself to PC2100.
For optimization: There really isnt anything, unless you had another stick of PC3200 (or the maximum of what your mobo can support).
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64