The Intel or AMD issue is solely by your personal preference.
I have built gamers with both and been satisfied.
Your PC2-5300 is actually 667Mhz in speed. At the present time, this memory is actually considered low-end memory for the types now available on the market. There are actually chips out there now with speeds ranging from 800, 1000, 1066, 1150Mhz.
As far as Dual-Core and Quad-Core,..this is a BIG issue with me. Most computers sold on the retail market are coming with Dual-Core processors. There is one thing you MUST understand. Almost all software on the market is single-thread (1 process at a time),..there are SOME out there along with some NEW GAMES wrote for dual threading (2 processes at the same time). To run 2 processes at the same time you need two "cores" or essentially two cpu's with "each" running a single thread. Do NOT waste your money on a quad-core. You will not use the other two left over until the software market 'catches up' with quad-core technology.
As far as the 4gb memory issue. First make sure to check if it is dual-channel or single-channel,..this makes a difference on the compatability of the motherboard also! Most new boards support from 2gb-4gb,..there are boards that support 8gb or more (such as mine),..HOWEVER, again, if your operating system doesn't support it you will NOT be utilizing the full 4gb.
Windows XP does NOT support 4gb's of memory usage. Windows XP max memory is 3gb's. Your computer will tell you 4gb's installed but will only utilize 3-3.2gb's of it. In order to utilize the full amount of memory you installed you would need to install windows xp (64bit) edition or Windows Vista to break the barrier.
Shop "wisely",..get hardware that is able to be "upgraded" to more current hardware to keep yourself ahead of the game when the time arrives!
Good Luck!