>I need to buy an internal hard disk for server at home. I am looking for something between 300 and 500 GB and am most concerned with performance since prices are relatively close. A larger disk will consume memory to hold the equivalent of the FAT map, right? The server currently has 1.25 GB memory and can go up to 2GB. How big a chunk of memory will a 500 GB disk require?

I don't think your assumption is correct.
I am under the impression that huge HDs require NTFS, and I understand that NTFS allocates it's "map" only as it needs it. So as the HD grows in content it will grow.

As reards consuming memory (I assume you mean RAM) I believe that is a transient thing. I believe Windows will use "all that it can" to meet its immediate needs but then allow it to be re-allocated to other processes as they demand RAM.
Since I doubt that you spend all day only 'exploring' the HD on the server I think your concern is unfounded.
The fact is, you have NO CONTROL (well, very minor control) over how/when RAM is allocated, for what and for how long. But I'd say that Windows today (XP at least) seems to do a reasonable job of memory management. If your performance is good now I wouldn't bother to change the RAM configuration until you see a problem.

By the way, to me the biggest single factor - assuming equivalent rotational speeds - is the number of platters. I will always go for the one that has the higher number of platters.
I make the basic assumption that an HD delivered with multiple platters has all of the platters in use, though it is possible that a specific HD may use only 1 side of a platter. Platters have 2 recordable surfaces but it is possible that only one surface is used.
The reason is simple - platters 'vertically' represent the cylinders of a HD and a HD will always fill 1 cylinder before moving to the next. So if a HD has 2 platters with all 4 sides in use there is twice as much data available to be read (or written) before the heads have to move to a new cylinder compared to one that has 1 platter using both sides. Obviously 3 platters is better than 2.

cheers
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>TIA,
>
>Alex