System memoryFollow

1. I don't think so.
2. Yes, the speed makes a great difference.
3. DDR is SDRAM where data is transferred at the top and bottom of the clock cycle instead of only on one end, effectively doubling the transfer rate. Do people even sell SDRAM anymore?
4. Yes. You will be able to load higher quality textures into memory. It won't run faster, but it will look nicer.

Any speed difference between having 2 1gb sticks or just 1 2gb stick?

-Yes, your motherboard supports dual channel memory interface (128bit), you will have a performance benifit with installing a matched pair in the correct slots (they'll be the same colour).

Is there much speed (or other) difference between the PC3200 and the 2700?

-Yes, your system runs on a basic bus speed of 400MHz, PC2700 is rated at 333MHz, PC3200 at 400MHz, so you have a discrepancy of some 66Mhz between PC2700 and your system. Which means your current RAM is creating a bottleneck.

Is SDRAM enough of a speed difference for the price over ddram?

-Forget about SDRAM, your system doesn't even take it and it's technology from last century, it smells funny.

It's probably a matter of economics whether you want to get 1 or 2 GB. Chances are about %100 your next new machine won't take DDR RAM so you're going to be stuck with it. If you can manage the expense there's no reason not to I guess.