RAM question

I have 512 MB of Ram and this is the next thing that I need to upgrade for my system.
Can someone explain to me what the code names/numbers mean to me, a thick consumer.
For example I am planning to buy two modules of the same type of Ram so that I get a gig and no conflicts from having differant sorts in my comp. What is the differance between:
512MB PC-3200 DDR Memory
512MB PC2100 DDR Memory
512MB PC-133 SDRAM Memory
512MB PC2700 DDR Memory
And so on.
Thanks.
Will
Can someone explain to me what the code names/numbers mean to me, a thick consumer.
For example I am planning to buy two modules of the same type of Ram so that I get a gig and no conflicts from having differant sorts in my comp. What is the differance between:
512MB PC-3200 DDR Memory
512MB PC2100 DDR Memory
512MB PC-133 SDRAM Memory
512MB PC2700 DDR Memory
And so on.
Thanks.
Will