Well the pc3200 is faster and is the general ram speed that is used today. pc2100 is an older ram stick which is slower and when used in conjunction with a faster stick of ram, will downclock the faster ram to pc2100 speeds.
The price of the pc2100 is probably higher due to rarity or something... So I suggest you get the pc3200.
A higher price doesn't mean that its better in most cases, but its a good thing you asked before you went and cheated yourself.
GIGABYTE GA-870A-UD3 AM3 AMD 870 SATA 6Gb/s USB 3.0 Motherboard
NZXT Apollo Black ATX Mid tower Case
AMD Phenom II X6 1055t @ 3.8 GHZ
Rosewill RCX-Z1 CPU heatsink
Sapphire TOXIC Edition Radeon HD 5850 1GB @ 800/1175
2 x 2GB CORSAIR DOMINATOR 240-Pin DDR3 1600mhz
G.Skill Sniper SSD 60gb x2 RAID 0
BFG Tech GS-550 550W ATX12V V2.2 / EPS12V V2.8 SLI PSU