These are the weird naming conventions I've never gotten a good explanation for (never really researched it either).

All the "DDR" RAMs are a completely different, faster type than the "SDRAM". If there were any consistency to naming conventions.. (PC133 runs at 133mghz) the DDRs would be renamed:

PC2100  would be  PC266
PC2700 would be   PC333
PC3200 would be   PC400 

as  266, 333 and 400 are the "mghz" they run at. ( I know.. another preposition ended sentence)

That's the best sense I can make of it.

You got it, Will. DDR stands for Dual Data Rate, and is a new very fast type of RAM. If you can, geddit . Also, get the highest speed possible. Therefore, 512MB PC-3200 DDR Memory. I would make sure you have enough space in your motherboard for the new RAM. If you have 512MB at the moment, this may consist of two 256MB chips. If they are, then you will only have one slot remaining, and you cannot buy two more chips. Also, if they are DDR, get DDR for the new RAM, and if they are SDRAM, get SDRAM for the new RAM.

One word of warning, RAM chips are extraordinarily static sensitive, so be extremely careful when putting it in, so you don't damage your new RAM, or any other bits of the gits of the computer that you might happen to touch.

Briefly, get exactly the same type of RAM that is in your PC at the moment. Make sure you have enough space, and most important, be very careful.

Jonathan 8)

Good stuff Will, as long as you have room for two new RAM chips in your motherboard.

Should make your FSs a little quicker

Jonathan 8)

Make sure what type of memory your system uses. Some systems can not use the DDR ram . And do not mix DDR ra, with SDRAM ram.   SDRAM stands for Synochrized Dynamic Ramdon Access Memory .

Will,

If your system currently uses DDR, then get DDR. If it currently uses SDRAM, then get SDRAM. It really is as simple as that

Jonathan 8)

What does DDR-SDRAM mean then?

You can see the break down on ram at www.pricewatch.com.
The post showing front side bus speeds and associated RAM is correct. 
I believe you can install 2700, 333 RAM in place of 2100, 266 and it will run at 266 MB FSB speed.  Another member needs to comment on this.  Older system SDRAM of 133 MHz could be installed in place of 100 MHz ram, a fact.  Advantage is that if a 2700 CPU is installed, RAM will match w/o upgrade cost.
Having gone to 1G RAM, I have found that none of the flight sims and OS use more than 512.  If I had to do it over, one 512 DIMM of CL2 RAM only.   
Best CL2 is by Corsair.  Mine is Kingston. 
The MB will recognize the high performance RAM and default set to correct BIOS values.