The problem right now is that all (or at least most) motherboards take a max. of 4 Gigs. This is likely to become a very limiting factor real soon. You should try to build your own system and get the best motherboard available. The difference between Intel and AMD is a matter of preference and price (where currently Intel has the edge), it will likely continue to see-saw between the two. The OS question is harder, currently it looks like Microsoft Vista 64 has the best future but given the resistance this could be replaced by some other 64 bit OS. Processors all have the capabilities but we will surely need a lot more memory with any future 64 bit OS.

Remember unless you play the latest games you don't need a top of the line videocard. Hard disks are very easy to replace or add (so go for something cheap). First priority is the best motherboard, next your CPU, then memory (you must not be surprised if the motherboard is the most expensive part of the computer).