I'm on some kind of intel quaddie with 4GB of DDR2. I run XP x64 which is kinda unusual but I like it a lot. I have a pretty good nVidia graphics card but nothing silly. I turn off my swap file now so its 4Gb or bust but its faster that way. I think that if I were building it again I would add a solid state drive to it and put some of the more frequently changing files on that. To be honest, if I were doing this all again I would go with Ubuntu 10.10 instead now as well. I have a somewhat old 4Gb dual core ubuntu 10.10 machine too with a silly graphics card which someone sold me for a song which I use as a test machine for some stuff I do but not generally mapping. I would urge all people putting together a major machine to go x64 now even if you don't have 4Gb+ of ram.

A removable drive bay is a kind of luxury but you can do just as good with external USB hard drives for backup. They are not quite as fast but I like the fact that for almost all of the time they are not connected to the main box and therefore unaffected by a terminal power spike.

I also have a UPS on my main machine too which I think is a bonus if you have moderately dodgy power. Its prevented a few hard stops in the past so it might have paid for itself by now.