The PC2700, 3200, 3500 etc refers to the maximum data transfer rate for that module. Unfortunately this is misleading (it was a marketing thing to make DDR look much faster than regular SDRAM, which rated in clockspeed (PC66, PC100, PC133), rather than data transfer rate.)
Basically:
PC2100 is DDR266 or 266 MHz DDR SDRAM memory.
PC2700 is DDR333 or 333 MHz capable DDR SDRAM.
PC3200 is DDR400 " 400 MHz
PC3500 is DDR433 " 433 MHz
All of these are the maximum clockspeeds the memory can run at. The PCxxxx comes from the fact that DDR memory at 133 MHz DDR (266 MHz effectively) can transfer up to 2100 MB/sec, this is why it's called PC2100. (Confused yet?
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CAS latency (meaning Column Address Strobe) is related to how DRAM type memory works. Basically DRAM memory is set up as rows and columns. When a CPU wants to access memory it needs to tell the memory modules what column and row (and bank, but we won't discuss that here) the data it wants is in. The lower the CAS latency, the fewer clock cycles required for the CPU to address the memory. All things being equal, CAS 2 memory will get the data to the CPU one clock cycle faster than CAS 3 memory. (DDR is usually CAS 2.5, or 2, with 2.5 being most common) You pay a big premium to get CAS 2 memory, but if you set memory timings in the bios to take advantage of it, your memory performance will be better.
Unless you are overclocking, there is no need to buy memory is capable of running at a higher clockspeed than required.
The requirements (for optimum performance) would be:
Athlon XPs up to 2400+ and including 2600+ models - PC2100
Athlon XPs 2500+, 2700+, 2800+, 3000+ - PC2700
Upcoming 400 MHz FSB Athlon XPs (3200+?) - PC3200
There are some additional things I could mention here about running the memory at a higher clockrate than the CPU FSB, but generally this actually results in a loss in performance.
