So if I've bought an Alienware notebook using a 512 stick of dual-channel DDR at 400MHz (PC3200) - what are my options as far as upgrading goes?

I've been finding better deals on 533 and 667MHz sticks - if upgrade to either one (or maybe a pair) of 1 gig sticks, will they clock down to the necessary 400MHz or am I looking at some other compatibility issues?

I'm open to any recommendations if there is anything I haven't thought of.

If your computer has DDR400 RAM, then there's no way that you can upgrade to DDR2 533/667 RAM. It won't work because DDR and DDR2 have a different number of pins on the bottom.

It sounds as though you are looking at ddr2. Make sure you are looking at ddr 533/677 and not ddr2 533/667 as this is incompatible. if it is ddr than it will work, but ddr based systems only normally run at ddr 400 as a max. that does not mean that it wont work, it will just run at ddr 400 speed, unless you overlock your motherboard mem speed. I have not seen ddr run at speeds of 677, and 533 is very uncommon nowadays. I have seen 500, 550, and 600 though. the 533/667 speed makes me think you are looking at ddr2.

Another quick question - can I mix/match brands when filling up the two memory slots? If I buy a 1 gig stick now, will I have to search around for the same brand/model later if I decide to add another?

You can mix and match brands; just make sure that the modules are the same speed. However, if you want to run in dual channel mode, you need to buy a pair of identical sticks.