What processor are you running?
It would be possible to run both memory modules at the same time provided you run your system at the lower front side bus speed (266MHz). It may be possible to run them both at 333MHz if the PC2700 stick is of sufficient quality, or if the RAM timings are dialed back a bit. No guarrantees though, if you get problems running your system at 333MHz that you didn't at 266MHz, you'll want to yank that PC2100 module and test again. .
Performance is dependent on what the system's front side bus speed is set to; if the system is at 333MHz FSB it will be faster than at 266MHz FSB, provided that the processor is running synchronous with the bus (that is, the processor is also running at a 333MHz front side bus). Some motherboards will let you run the memory asynchronously (a 333MHz FSB processor with 266MHz memory, or vice versa) but running RAM at a slower speed than the processor results in lower system performance, running RAM faster than the processor results in negligible gains in speed (occasionally 1-2% faster, but sometimes 1-2% slower on some mainboards). You cannot run individual memory modules at separate bus speeds, they must be both run at 266, or both at 333.
I'm running two PC2700 sticks at PC3200 speeds, but I bought Corsair XMS memory at the time; good quality memory will often operate one speed rating above that which it is sold at.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890