DDR would be best option - then buy more if you need it. Most applications can get by happily with 512MBs of RAM. Buying one stick (512MB) will offer more legroom for upgrading later, but 2 sticks of 256MB could work out cheaper and offer dual channel capabilities if your motherboard chipset supports.
However, you should use the crucial website click here (select UK) to check whether your motherboard can support DDR ram. If it can, then it's a question of what speeds it can support. The base speed is PC2100 (266MHz) but it may support either the next one up PC2700 (333MHz) or even PC3200 (400MHz). Get the fastest available supported by your motherboard.