Your decision to go with 64-bit now is a good idea as that is not something you can incrementally upgrade later on; you can't just buy another 32 bits to add on to the CPU. However, RAM is something that can be done incrementally. More RAM is definitely better. But if the price of RAM drops, you may be paying too much now for what you only really need later.
I build my own computers. Thus I know how many RAM slots I have. I always make sure I start with no more than 50% used. Usually computers have 2 or 4 slots to install RAM, but some have 1 or 3 (a few I've worked on had as many as 16). A few I have with 3 slots can't use the 3rd slot if the largest sizes are installed in the first 2.
Anyway, to the extent you can do so, be sure you have exapandability. Let's say your new machine has 2 slots. You could get the 512MB as 2 modules of 256MB each, and fill it up, leaving no room for expansion, requiring you to replace both later on to go up to 1024MB. But if you get it with a single 512MB module, it takes one space, and leaves one space open for adding another later.
Beware about computer vendors stiffing you on this. Sometimes the larger RAM modules will cost a wee bit more than 2 that are half the size ... and sometimes not. But they will prefer to stiff you with 2 smaller ones for 2 reasons. One is that it forces you to buy more later when you want to expand (drives more business). The other is that it helps them use up what is going to be old stock soon as the smaller modules are no longer of interest.
This is one of the reasons I build my own, even though it now days actually costs a bit more. Not only do I know what's actually inside this way, I control precisely how it is configured. Of course I am at an advantage since I've been building PCs since 1993 and have been working on computer hardware since 1979.